Yeah, multiplayer is definitely a very tricky chicken and egg problem. Do you have any data on whether the situation is better with Steam users vs non-Steam (ie. are more percentages of Steam users active in MP vs. non-Steam)? If so it may be an indication of the issue of coordinating matches, maybe.
Perhaps the $ you've earned so far on Clones may allow you to, at least for a short while, hire a pool of people whose FT/PT job is to host public MP sessions and invite others to join? They don't even have to be good MP players, if anything that might be a hindrance.
The theory is that maybe once you get enough of a nucleus of people hooked on MP, then at some point hopefully it becomes self-sustaining and you no longer need to continue hiring the artificial pool of people to maintain an active MP community of players.
Another way to go is to see if we can work out some other way to incentivitize people into doing MP besides just the sheer fun of it. No ideas from me so far, but if I think of anything I'll post here.
[edit: in the meanwhile, I guess more jams won't hurt and is certainly cheaper
. Are you able to see from historical data whether the jams have the desired effects (in terms of getting more players more active in MP) and for how long the effects last?]