A social network for bird watchers???
I don't know, maybe they are so passionate they would pay for membership. It just seems like a niche inside a niche inside a niche to me.
As for what they buy, they do buy a bunch of things like books, binos, recorders, clothes, travel to those places etc. etc.
I have no idea if this particular TC50 company will execute well. And even if they do, whether they'll make any money at all. But you seemed to be questioning the very premise of such a site and I think you should reconsider that. Birding is not that small a niche within a niche... Perhaps a site for Latvian poetry that was written after the Soviet Occupation but before Gorbachev - that's a niche! Not birdwatching.
I don't know about birdwatching... maybe everyone apart from me is doing it and spending a lot of money on it :)
When I was into it, I bought a few books, a nice pair of binoculars, took a trip to a National Park in the north of India in the middle of winter and often woke up at daybreak to watch birds. That last thing required real commitment. I guess we'll leave it at that.