The "official" rideshare board has had a good many technical problems over the last few years. There is, however, an "unofficial" backup : try this. Registration is required.
If you have any questions, I'll be glad to answer them over on the Travel to Burning Man board. Having spent more time dealing with the trollage on this board than I'd care to, however, I'm probably going to treat this post as a drop and walk, so don't expect a response here.