I signed up to Dopplr, a social networking site for travellers, a good long time ago. After the to-be-expected initial flurry of activity, I can count on one hand the number of times I’ve logged into it. Most of those times were because a friend had noted that they were making a trip to Glasgow in the near future (it was the same friend on every occasion). To me, that’s not a particularly good amount of usage.
I think one of the biggest problems that Dopplr faced in its early (and critical) stage is that it is yet another social network at a time when people were starting to grow weary of signing up and amassing their friends again. Myspace and Bebo had already started to become less commonplace, and Facebook was taking over. Given that a lot of people viewed joining up to a general social network as a potential hassle, the chances of them joining up to a single-application social network was not going to happen.
That’s the lesson here: if you have only one particular domain of interest (whether travel, music, art, business etc), don’t try to build a social network around it. It takes a lot of people to get these things moving and by the very essence of operating in a niche, you are extremely unlikely to succeed in capturing much attention.
A better plan: let people use an existing social network to use your application. There’s much less friction that way. You only have to convince people that you’re worth their time, rather than having to do that _and_ convincing them to part with their time to help your application work.