Welcome to the world of Spam.
No, they haven't hacked your site. It's just spam bots that are smart enough now to get around your spam protection.
Upgrading to the latest version *usually* helps, but there is a long thread around here somewhere about spam bots having field days and posting hundreds of posts on the latest vB version.
Do your members verify their e-mail address?
Do you have any spam protection turned on yet? (reCaptcha, question/answer, etc.)
Have a look in the
mods section - there are some good anti-spam mods that will take care of your spammers for you.