Right, legally it's okay for facebook to censor stuff. But ethically/morally, some people do not think that's the case because they feel they're abusing their large marketshare.
Ethics and morals with Facebook? They have a long troubled history of dealing with both. It's a company, and companies want to make money, that's the bottom line. Of course it's censored in their best interests.
On one hand Facebook is a private company. On the other they are built around a communication platform and communication is a constitutionally protect right in America. Moreover, this type of practice could be considered anti-competitive and subject to anti-trust regulation (a bit unusual, but there is precedent).
I am not trying to advocate one or another view on this issue...more of trying to frame what I think is an interesting question: Where is the line where private rights give way to public rights...if at all?