I suspect you've posted your query elsewhere, and cut and pasted the posted query as it appeared on the other site, thus capturing the vBulletin-style abbreviated URL isntead of the real URL. (You have an interesting presentation of vBulletin on your site, by the way.)
I'm not going to give you a direct opinion on your situation, although others are free to do so, as to have a specific case analyzed you really should consult a lawyer. But in general terms, the owner of a public online bulletin board enjoys significant protection under the "safe harbor" provisions of the Communications Decency Act. That Act, passed in 1996, states "no provider or user of an interactive computer service shall be treated as a publisher or speaker of any information provided by another information content provider." This has been applied by the courts to provide broad protection to website owners for defamatory postings made by users. (Here's a Google search which brings up a bunch of related information.)
The users who post such information have no such protection.

