Posting in the hpalternity community is a way of signaling that your post ought to be read by more people. What the magic defines as "more"? Probably the community you live in, whether or not they're your friends. But who knows? It's a vague and magic-ridden category, and while Voldemort's people would like to say that the alternity community's posts are pushed to everyone, they can't tell you why or how.