I think your argument is begging the question as it depends on a prior belief that Facebook is actually harmful. Replace Facebook with a gym membership, or a charity, or some other thing that one does not presume is harmful as the premise of their argument.

Would the leadership of a gym be compromised and hypocrites if they banned someone who used their gym to tell others that working out is harmful and they should stop exercising at the gym? Would banning that member indicate that the owners of the gym don't care about the health of their users and instead put profits over well being?

Your argument only works and sounds credible because most people on this site believe Facebook is harmful. If people don't share that belief, then it doesn't follow that Facebook is being hypocritical because they are banning someone who is using their product to defeat the product.