You can't keep blaming Facebook and Twitter because the majority of the world has the attention span of a fish (which these two specific sites cater for). I'd also throw in <insert what you you just had to eat> and throw up a picture of said food, not the type of audience you really want. Accumulating members is the easy part, keeping them on your forum and getting them posting is the tough part, if you give them something that they can engage in then they'll come, possibly stay and remain active, remember Rome wasn't built in a day.
Point being, the sucess of your forum is down to you, not Foodbook and if you feel you can enter a niche that you can provide indepth, creative and unique content for your members then often than not paid software is where you want to be looking at since they usually provide the best support and 3rd party products.
You make the choice, nobody can do that for you from when I used XF. the Software speaks for it self, alone.
Have you tried vBulletin 5s? I hear that is coming along quite beautifully?