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.