On a related note, I've been on the partial Twitter firehose and I've only seen around 5 million unique users over a couple of months. Granted this is the partial firehose, but I'd still expect the number of uniques to approach the true number of active users over a long enough time period. I'm seeing comparitively few new users at this point.

One other researcher I've talked to has similar results, anybody else gathering data on this?