Why beta? You released some projects while you feel them aren't quite ready to go, so you can tagged "beta" because they aren't prefect for users yet.
Why beta? It's related with some known web 2.0 principles, liked "release early", "feedback quickly", "update frequently" and so on, because there is no site can meet users' requirement exactly, there is always changing, changing, and carry on.
Why beta? That means you promised you will keep tracing your user's feedback and improving your service, sure, that equals promises.
So, you can labeled your site "beta" early, but if your site doesn't meet the web 2.0 principles especially "update frequently", please removed the label, otherwise like Fred said:
"Just don’t label your web applications beta."