They aren't buying the revenues, they are buying the user relationship data. Who refers who, and for what products, in what locations. It's cleaner data than FBs (by a country mile), and already commercialised. Whats not to love?
As a side issue, I see people talking about FBs 500m users. No. FB has like, 100m users, and 400m bots and fake profiles (ok, maybe that's a slight exaggeration, but you get my point). I know for certain that the hot, blond, bisexual chick I'm "friends" with, is actually a fat bloke
. Most of "her" friends have no clue