I doubt you can make money with forums posting.this day many webmaster irritate with so many forums poster that only posting their post using xrumer and degrade their forums value infront of Google SERPS
I don't think having a link in a signature gives you money, it may give you some extra traffic which normally bumps up your bounce rate but apart from that it is just about building relationships with other forum members.
I don't come on here often because I have mainly one or two other favourites but how someone could keep up with many more is a little beyond me. It must say that somebody out there makes money from them.
Making money with forum signatures is not that easy, many firms offer this service, but of course it is possible. SEO is always in demand and most everyone knows that links are the way to drive traffic to a site.