So I haven't read a lot of this thread, but I can talk about something we've discovered on Lost & Found that's been pretty successful for us: Tumblr. We're in a position where our game canon -- not just Transformers, but specifically the comic More Than Meets the Eye -- has a huge fanbase on Tumblr. We've had a game Tumblr since inception. At one point we got a player who was experienced with MU*s but also active on Tumblr and the TF fandom there, and they started poking around RPer accounts on there and inviting people who seemed promising. I mean, Tumblr already seems like an awful platform to RP on, because it kind of makes conversation really difficult, and I've always believed that these people would probably be open to other options if presented to them. So now we've had a whole bunch of players who came in from Tumblr and learned the mechanics of the game, and we suddenly started exploding in size.
Here's the catches:
You have to be exceptionally newbie-friendly. We've been rolling out super basic guides to a lot of stuff both on the wiki and on the game. A lot of these folks have never heard of MUs, but could become awesome MU players as long as you're open to hand-holding for a while. Honestly, we've gotten some players for Tumblr who have picked up things faster than veteran MU*ers who can't for the life of them remember certain commands they use regularly.
We had to bring on more staff, and if we didn't have players willing, we would be in a position where we couldn't handle things at all. We went from a three-person staff to something like eight (although those additional folks mostly in specialized positions). If we didn't have people who were willing and able to step up and run plot and process apps, it wouldn't have been successful.
So I guess the moral of the story is: recruit outside the usual places. Find your audience if you're not getting enough from the MU* population.