There's a point in social media best termed "critical mass". Before that certain, yet unknowable, number of users, "nobody" is using the service because they think nobody else is. After the critical mass point, "everyone" uses the service, because "everyone else" is doing so. This is as important for Facebook as it is for Wikipedia.
Some of my forebears at my former employer had tried setting up a wiki for the sharing of knowledge. It was not entirely smooth. Some people misunderstood the purpose and requested that certain people be given only read access, or that they have a separate wiki for their project. Mostly people didn't bother trying to read anything there because too few people were writing anything. To get it off the ground, we needed a lot more people devoting attention to this effort as part of their work day. The point is that it's tough to get started, but once it's done, it is well worth the effort.
Mokalus of Borg
PS - Encouraging early contributors means finding them before they have contributed.
PPS - I have no idea how to do that.