A magnetic site will also have spread contagiously, and will stick like velcro. The key difference with magnetic sites is that the users benefit more as more people become connected. The more people you can magnetically attract to your group area, the greater the added value to the users. This is synergy. The users and site owners both benefit.
Users have the added benefit of connectedness with like-minded persons.
Site owners can tap into new advertising methods. In fact, extremely narrow advertising channels can be created by targeting specific groups. One of the primary strengths of magnetic sites is that, once a group finds its homw on the web, it is not likely to leave it. It's like having a regular group of locals who haunt the town diner for coffee every morning. These people keep coming back.