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Some recent headline-grabbing cases of stalking range from frightening to downright horrific. But there's also a subtler form of stalking, one that doesn't require physical proximity: online stalking.

It's one thing to occasionally check the Facebook page of an ex-boyfriend or ex-girlfriend, but it's another, says psychologist and addiction expert Seth Meyers, to obsessively track someone on the web.

Here are three signs you might have crossed the line and should consider getting professional help.

1. "Spending a lot of time trying to get as much information as possible about the person," Meyers said in an interview with ABC News' "20/20." Are you missing deadlines or canceling social plans because of long Web-surfing sessions and-or constant monitoring of the person's social media posts?
2. "Your stalking behavior is done in secret," Meyers said. This means lurking on sites, leaving no trace you were there, like a comment on a post, for example.
3. "Using multiple social media sites to get as much information as they can," Meyers said, characterized as a "hunt" compelled by a deep-seated insecurity. "The root of it is about feeling that this other person has something that you don't," he said. "You'd feel more complete if you have what they have."

Watch the full story on "20/20: Stalked!" online now.