I heavily doubt hat fetlife makes as much money as, say, Google. At least it doesn't don't show up in the worlds top hundred most valuable companies.
What I don't like about fetlife is that I have to deal with idiot's messages all the time and that people who send me an unsolicited friendship request actually expect me to explain why I trash them.
One of the blokes from where I live said "I want to follow you" when I asked him why he sent me a friendship request. Oh yeah, thanks, but no, thanks...
Apart from all that: Yes, there is no substitution for real life. But online can get amazingly real, too, especially for a imagination junky like me.