As of this weekend, Yelp has officially logged 100 million reviews on its site -- something that chief executive Jeremy Stoppelman said he couldn't have dreamed of when he first started the company as a clunky email recommendation service.

The option to write your own review for a business was an afterthought in the original version of Yelp, which had users seek suggestions for the area's best doctor or a reliable plumber from friends, over email.  "The first idea was a service asking friends for recommendations, and buried in there was this option to write your own review," Stoppelman said.

After all, it was 2004 -- there were no check-ins, no social media updates, and no smartphones -- and there was certainly no assumption that you could depend on the wisdom of the crowd to find a good lunch spot. Besides, Stoppelman thought, why would anyone take the time to write up a review for strangers?

"I really had this conviction that no one was going to write reviews for the heck of it," Stoppelman said.

Boy, was he wrong about that.

Eleven years and 100 million reviews later Yelp has somehow managed to take one of the most intimate interactions, the personal recommendation, and make it the foundation of a global $1.6 billion company. Along the way, Yelp's also benefited from some of the tech industry's most disruptive trends, such as the growing importance of location data, the shift to mobile devices. Perhaps most of all, Yelp benefited from the realization that -- for better or for worse --  people really, really love to share their opinions about everything online.

That includes, of course, Yelp itself. In its pursuit of growth, the company has formed a complicated relationship with business owners. Some love the visibility Yelp gives them. Others have gone so far as to accuse the site of manipulating results for various reasons -- though no judge or regulator has ever found concrete evidence to back up those accusations. The company was also recently and publicly criticized via a scathing review from a now-former employee who lost her job after criticizing Yelp for paying employees salaries that are too low for San Francisco, and for not giving employees enough chances to move up the ladder.

That uproar particularly took the company by surprise, given that it's tried to maintain a workplace culture that reflects the democratic nature of its site. Stoppelman, for example, famously doesn't have an office, just a desk among other Yelp employees.

"Our team thinks every day how to get people to where they want to be. It's top of mind for us all the time," said Erica Galos Alioto, senior vice president of sales at Yelp, who said that the company works hard at retention, and often tries to shuffle employees around to find a good job fit. "Given that, it never feels good to hear that somebody’s experience or perspective is counter to that. That being said, it’s also important to recognize that you can't keep 100 percent of people happy all the time -- that's something we tell business owners as well."

Still, with 100 million reviews in 11 years, it's clear that Yelp has hit on some sort of formula for success. The 100-millionth review was of a neighborhood park and was written by Evelyn K. -- a member of the site's exclusive "Yelp Elite." This select group of Yelpers aren't employees of the company but are key to its central goal to "be authentic" by  keeping it grounded in local communities, even as it pursues aggressive growth.

To become a Yelp Elite member, you have to be nominated -- by yourself or another person on Yelp -- and are then reviewed by a secretive company board that evaluates whether a nominee posts content that people in the area find useful. Elite members also have to be over 21 years old and have to use their real names and pictures for their profiles. Once you're Elite, you get a special badge on your profile and access to special events in your city, and you stay in close contact with your area's Yelp community manager, who is in charge of scheduling events and working with local businesses to promote the Yelp community.

The number of Yelp Elite members varies, said Colleen Burns, Yelp's regional marketing director for the southeastern United States, but the number of dedicated Yelp users has grown. When Burns started as Yelp's community manager for Orlando in 2009, she said, an event she hosted had about 45 attendees. At the last event there, she said, there were roughly 1,200.

Burns credits the community manger system for helping Yelp retain a lot of its personal touch over the years by handing over community management to people with the right attitude to make the review site work. "There's no good formula," she said. "But you have to have passion. It's always easy when somebody loves the city and they're into connecting movers and shakers."

And these movers and shakers are more connected these days than ever before. The ongoing shift to mobile devices overall has also affected how Yelp functions as a site. The company was among the handful of businesses that launched an app alongside Apple's App Store in 2008, and now 70 percent of the company's searches are done through the mobile app.

"What mobile meant was you could articulate location in a much more meaningful way than when you were on the desktop," Stoppelman said. Rather than having to type in your address or neighborhood, he said, the advent of location services meant that Yelp could instantly recommend things around any given user -- and that it has become much easier for users to post reviews as well. There are also more pictures in reviews, which gives users a good sense of ambiance or cleanliness.

That's changed some of the ways that reviews have come in. Mobile reviews in the nightlife category, for example, spike on the weekends -- making it seem as though people are actively posting about clubs or restaurants while they're out. For the Web, however, people tend to sleep off their revelry and send in reviews on Monday morning. So if you're looking for somewhere to go while you're out, you run a good chance of finding a fresh review even on-the-go.

Yelp has changed some of its procedures for vetting posts with the rise of mobile, too. People can write reviews on their phones but are encouraged throughout the process to keep writing -- a check against quick, low-quality reviews. Yelp has also added the option to add a "quick tip" on mobile, for those who really do just only want to recommend a dish, compliment a server or warn people off because of a dirty bathroom.

But not all categories are affected by mobile in the same way. For example, people tend to write on Web and mobile at the same times when reviewing physicians -- and to take their time writing them. Reviews come in at roughly the same times on mobile and Web throughout the week.

Analysts have recently dinged Yelp for focusing on growth at the expense of profit, while also raising worries about its ability to stand up to the competition. And Yelp itself announced last month that it would replace its chief financial officer, which many took as sign that it would not hit its goal of $1 billion in revenue by the end of 2017.

"Yelp’s biggest innovation was combining a user review site with community features and a modern desktop and mobile interface. It understood that anonymous local reviews are valuable, real-name reviews are better, and reviews by those you actually know are best," wrote Pacific Crest analysts Evan Wilson and Tyler Parker in a recent note to investors.  "Unfortunately, Yelp's user and advertiser trends show that this use case is more fully penetrated than it used to be by Yelp and its competitors, and growth will likely decelerate meaningfully or Yelp will have to spend more to prop up growth."

Looking ahead, Stoppelman said, Yelp will evolve to focus more on enabling transactions through the site itself rather than simply acting as a guide. That way, if you want to book a handyman, you can search for a good one on Yelp and then request a quote straight from the site.

"There will be more of that delivering the interaction with the business owner," he said. "Then it’s up to you to engage with that business."