Taking a stance against a topical issue will get you a big audience at great speed. The biggest and fastest responding groups in history have been formed in opposition to an existing issue. Tea party protests, French rioters, we see examples every day.
The problem is the connections between members are weak. The quality of conversations are poor. People don’t get to know each other. They just unite, briefly, to fight against a perceived injustice. They have an end in sight.
Once they reach that end (or seemingly fail to reach that end), these groups dissipate. You’re left with little more than you had to begin with.
It’s far harder to build a group who want to interact with each other for a common goal, as opposed to correct something wrong in the present. You have to slowly bring people together. You have to sew these relationships together. It takes time – but it lasts longer.
Go negative for a big, weaker, audience or go positive for a smaller, stronger, audience. Your choice.