LinkedIn already does this sort of, which is annoying. People can see you browsing them if they also agree to have themselves be seen. Tit for Tat. I believe there is also a monetization component but I don't know the details. I think some of the paid plans allow you to see who is viewing you regardless of their own setting.

Though somehow I feel it's less creepy for "professional" sites vs. personal sites. I don't really care about coworkers looking for me, or if I'm looking at coworkers. But if it's friends from 10 years ago it is awkward.

Yeah I have no problem with LinkedIn doing that because they are way more about making business connections whereas Facebook is a much more personal thing where I think people like to keep their actions more private.

Haha, OKCupid does that, you can pay for "Alpha" status that means you can stalk people's profiles without them knowing.