This is Genius because Linkedin has been working really hard on one feature that could possibly make a lot of users upgrade.

The fact that we always want to know who "peaked" at our profiles (I'm sure half of your secretly wanted to know at some point of your life who looked at your facebook profile right?) leads people to upgrade.

A funny thing happened to me a few months ago, I was browsing linkedin and landed on a old client's profile and I was wondering what was going on with him and his projects.

A few hours later I got an email from him saying that he got a notification that I was viewing his profile and that made him wondered what is going on with me (and our company). That interaction made me think that this single feature is genius, it helps reconnect with people (some will argue that it might be creepy).