Well, even with Facebook groups it's a good idea to pay to get members - but you don't have to pay them to keep posting! However, though, reaching targeted traffic on Facebook or some other quality source - can lead to signups of real people who are more likely to post - not only on your group, but possibly your forum too - and, of course, I mean for free.
Anyway, I don't mind keep people around to post for money - knowing they will stop when the cash stops. It's not a put-down of my website. However, paying people all the time - can be quite expensive and many cannot afford it!
I have never paid anyone to be a member on any of mine over the years, what is the point? People join cause they want to join, if you paying someone to join, then there is something wrong stopping people from joining in the first place.