As a flood of family photos, videos and holiday greetings hits the Internet this time of year, online users will be swarming the social-networking and photo-sharing sites that have become the personal scrapbooks of our time.

But in the shaky and often promiscuous business of the Web, where companies fold and merge at astonishing speed, you can’t always trust that those sites will take long-term care of your digital treasures.

Gowalla, a service that lets users announce their whereabouts, is set to close down in weeks. In October, Google canned its social network Google Buzz. The Web’s first major social network, Friendster, was overhauled in June to focus on video games.

In many cases, the data that people have entrusted to such sites exist in a cyber limbo, and users’ rights are unclear.

“People have very personal relationships with the places where they are putting their personal information,” said Leslie Harris, president of the Center for Democracy and Technology, a public interest group. “But your right to that data becomes more complicated with the Internet and the cloud,” the system of shared computer storage that allows users to tap into the Web on the go.

The details of a user’s rights are often embedded in long legal policies that federal regulators complain are often too confusing and seldom read.

And they are no solace to users like John Metta, 41, who has experimented with many Web sites that have come out of Silicon Valley’s booming social-media industry.

In the past year, Metta has joined Twitter, photo-sharing site Picasa, Gowalla and Google’s new social-networking service, Google+.

He’s used Gowalla to “check in” at coffee shops, bars and concerts in Portland, Ore., alerting friends who might want to join him. He’s shared pictures of his wife in front of sweeping vistas near Hood River, Ore., part of an online timeline the site created that shows where he’s been and who he’s met over the past year.

All that information will be deleted after Gowalla’s employees begin working for Facebook to create a new location service. The company said it will notify its 2 million users about how to download contact lists, photos and other information.

Metta, a 41-year-old software designer, is a bit burned out from trying to maintain all that information online. “At some point, you have to just let go and be Buddhist about it,” Metta said.

Letting go becomes harder when some of the Web’s biggest sites face uncertainty. As Eastman Kodak tries to shore up its finances, questions surround the fate of the photo albums created by its 75 million Kodak Gallery users, some analysts say.

After acquisitions, users of Flickr saw their albums moved to Yahoo, and Picasa users’ photos were migrated to Google. Those sites were able to fold customers’ information into their other services for targeted advertising. If Kodak is acquired, rights to its Gallery photos will go to the new buyer, too, according to its online legal policy. A Kodak spokesman declined to comment on the company’s business plans and referred questions about user rights to the company’s privacy policy.

That policy says users have the right to request that images not go to another company.

But there are no standard privacy rules for Web sites, federal enforcement officials say, and consumers encounter many practices.

Friendster e-mailed users about its planned changes and suggested ways for them to download their data. The company, owned by the Malaysian firm MOL, said it won’t delete customer databases.

Google also told users that it would shut down Buzz and that it doesn’t plan to delete information in those accounts.

Consumers might find even bigger surprises when an entire business fails. Bookseller Borders this year auctioned off its customer database, including purchase history, in bankruptcy court. Information about users on the defunct gay youth Web site XY were put up for sale, too, until the Federal Trade Commission stepped in to prevent the sale.

Federal enforcement officials are stepping up efforts to protect online privacy as change comes fast.

“There is no general legal requirement for companies to get rid of information,” said Christopher N. Olsen, the FTC’s assistant director of privacy and identity protection. It recently charged Facebook with failing to delete past users’ data, even though it said it had.

“There have been more and more issues that require our attention,” he said.

That all comes as news to Rashida Isigi, 34, a New York-based corporate recruiter who understands the risks involved in her online activity.

She surprised herself recently when tracing back her online history by counting the various sites she’s joined. Isigi didn’t realize she was littering the Internet with so many pieces of personal information.

She’s had several e-mail accounts, through AOL, Hotmail, Gmail, the University of Arizona and eight corporate addresses. She’s on MySpace, Facebook, Twitter, LinkedIn and HootSuite — which lets her post comments to several social-media sites at once.

Most of the accounts have been idle for years, but she hasn’t shut them down or deleted her information. She said she accepts that there’s a price to pay for using the services.

She checks her Hotmail account once a month just to clean out the mailbox. And she rarely thinks about her MySpace profile, created during her late 20s.

“At some point, you just have to surrender control,” she said.