In this space, I’ll occasionally explore topics proposed by readers. This question recently came up in my online chat.

I was feeling nostalgic, and I searched online for my college boyfriend, whom I had not spoken to in about 15 years. He was always an off-the-grid kind of guy, so it took about 30 minutes, but I found a cellphone number that I’m fairly confident is his. Would calling him be harmless fun, or weird? I only want to say hi and hear what he’s been up to. Absolutely nothing more.

The classic game: skilled researcher or creepy stalker?

In assorted online polls, the percentage of people who have searched for lost exes has hovered around 75 percent, meaning that one of the more fascinating things about the Internet is the way it has made “I am stalking you” into an acceptable greeting rather than a reason to use a rape whistle.

From the morass of online behavior, incontrovertible rules have emerged:

A Facebook search is always acceptable — the site’s existence is based on stalking the people we’ve lost touch with, remembering why we lost touch with them and then quietly hiding their feeds.

A quick Google search is also fine. It’s an easy way to see what old acquaintances are up to (“Still live-blogging ‘Toddlers and Tiaras,’ I see”) and plan accordingly.

Beyond that, the difference between “catching up” and “mailing someone an envelope of your fingernail clippings” is all math and algorithms, dependent on the following factors:

1) Have you ever slept with the person you’re now tracking?

People trying to locate distant relatives are not seen as stalkerish, because this activity bears the respectable name “genealogical research.” But the stalk-o-meter for locating exes is easily triggered, because it’s saddled with ulterior motives: Why do you really want to talk to her? What’s missing in your life now?

The amount of time you spend looking for an ex should be directly proportional to the importance of the reason you want to find them. “I couldn’t stop thinking about his left nostril” comes across as creepy. “Our mutual friend has cancer, and I thought he’d like to know” does not.

2) Are you using advanced search tools that would imply you have a high-security clearance?

Although you might have access to databases such as LexisNexis for your job, using such a tool for personal searches implies a potentially icky level of investment. If someone is not on Facebook, he or she is not trying to be found. If you find the person anyway, it signals that you put forth extra effort. You must be prepared to answer the nervous “How did you find me?” in a reassuring way that does not involve the phrase “this lovely computer hacker I know.”

3) What modes of contact are you planning to use?

In increasing order of creepiness, the various methods of reaching someone are: Twitter/Facebook, work e-mail, personal e-mail, work phone, home phone and cell number.

E-mail is less invasive than the phone, because it gives your target the choice of whether to respond and the time to prepare a response. Work is less invasive than home, because it’s usually less guarded: Many businesses and groups have Web sites that include this kind of contact information.

So, back to the question that started this discussion: You spent 30 minutes using elevated research techniques to find the cell number of an ex you haven’t spoken to in more than a decade, who has deliberately made his online presence scarce.

I think that could come across as creepy. But I don’t know the guy. You do, or did, so you’re a better judge of his appreciation for serendipity and whether or not he’d think you were a nut job. More important, you’re a better judge of whether or not you’d care whether he thought that.

If he is put off by your efforts, what’s the worst that could happen — he disappears for 15 years?

Feeling vexed or perplexed by life online? E-mail hessem@washpost.com with a question or visit the Web Hostess weekly chat at 2 p.m. Wednesdays.