[parker]

It's simple user psychology. So much of traffic on social networking sites is lurking around on other people's pages, especially those that you're 'interested' in. I'd rather not let other people know that I've been looking at their page, especially when I've been looking at their page a lot. If I know that they know, it dis-incentivizes me from surfing around at all, killing page views.

In other words, social networks need to do everything they can to nurture their creepers.

[nostrademons]

FaceBook had a huge blunder a couple months ago when it seemed like typing a space in the search field apparently autocompleted the people who most recently looked at your profile page. It was never confirmed that that was what was happening, but people were massively creeped out and they fixed the bug within a day.