This is no different than a local (physical) bulletin board.
There IS a difference. When you post on a local bulletin board you walk up to it in person and stick the paper on the board. There is risk to your anonymity and there is also more effort required. A barrier to entry, but one not so high that good-intentioned folks will be stopped by it -- just enough to keep the lazy trolls off the bridge.
And am *I* the one who gets to decide what is and isn't? Or you?
Yes. That's how democracy works (or is supposed to). No individuals decide, instead we all do as a society. Yes, that means the decisions won't all go your way.
Saying the wrong thing that offends someone on a college campus these days will get you kicked out faster than banging Dean Wormer's wife.
What's wrong with that? Freedom of expression doesn't have anything to do with freedom from the consequences of being a jackass.
To do nothing is to be nothing.