While, I'm W3C staff contact for the working group now, I wasn't involved as staff when the group started, so I'm not sure exactly what happened. By W3C practice, ever member organization is consulted on every new thing we do, though, so Facebook, Google, Twitter, etc were informed and asked for input and invited to participate. The public is also informed, although I don't know how much people pay attention to W3C news, and the framing might not have caught the right people's attention:
https://www.w3.org/blog/news/archives/3958. I still have a hard time understanding what that announcement is talking about.
In any case, as other people have said, perhaps these companies didn't see the business case to participate. This baffles me a little. I can see why as the market leader Facebook wouldn't necessarily benefit financially from helping the effort, but if I were them, I'd want to at least have someone in the room, paying attention. And if I were anyone else, I'd probably see the value in teaming up and helping. But that's not what happened, so I'm clearly wrong about something. It's possible they just thought the problem was too hard and the group would never succeed, so it wasn't worth paying it any attention. My sense is it's done well enough that with more help it might have done great. But it was touch and go at many points, and I personally thought it would go nowhere until our Paris meeting, about 8 months in. It's also possible they just didn't have anyone senior enough to handle the job who had any desire to do it.
And yes, OpenSocial were one of the big motivators for this work starting at W3C (as the news announcement says). But, aside from IBM, they never actually showed up in the group. Someday, I'd like to understand that, too.