See if there are any local Jam Nights in your area, also check advert boards at your local music shops. If you want to find something online, JoinMyBand is one I used like you, but also try BandMix and, like you say, have a gander on Facebook for local groups of musicians etc.
I started going to a new Jam Night last year and have met so many great and like-minded musicians, it's a brilliant way to meet people but also see what kind of musical ability they have in a live situation before proposing forming a band. :)