For us, the 20 people will usually yield 2 or 3 that are interested. Of those, we'll usually get to meet 50% of them, and if we get to the point of actually meeting, it's easily 50% or better that we'll wind up playing.
Biggest thing that gets in the way for us is simply schedules. Just trying to find a date/time that works for both can be tough. We had a case where we had one couple we were supposed to meet and their babysitter flaked at the last minute. We still haven't me them, but there's little doubt we'd play if/when we do.
It also makes a big difference on which way the communication is going. If we get a message from an interested couple and they look and sound decent, in most cases we're probably at better than 50% that we'll meet and play. Only snag there is when we get the message and it's the guy and says that he needs to talk to the Mrs first. Probably 80% of those are what I suspect are wishful thinkers and I don't usually hear back.
You'd think it would be way easier to meet and play, but it's simply not. That's why we also don't put a huge effort into meeting people online. If it happens and works out, great, but normally parties is where we meet most of our new playmates.