The distribution list has long since come and gone - we generally post up here if there's an open invite ride going on.
There are a few of us here in the eastbay - I can't speak for the others, but if I'm riding locally, it's on the roadbike. I think Harpo is the same way. Most of the eastbay folks I ride with are also roadie / mtb crossovers. Fireroads don't do much for me, and the only legal singletrack (Miller) is a little tight, with lots of hikers.
Have you done both sides of China Camp? That's probably the most common mtb spot for us eastbay folk. Most of us also have season passes for Boy Scout Camp as well - generally, we either do two loops to make it worth the gas / toll to get out there, or combine it with a longer loop of China Camp.
Just curious, what sort of bike do you ride, and how long are the rides you typically do?