On their home page CB currently claim 1,977,762 visitors across all sites. We see the following across our network:

397,727,080 visits per month.

1,002,792,862 impressions per month.

209,727,427 absolute uniques per month.

We use a third party to report this data to ensure objectivity when we're doing reports for outsiders. This is from the period March 30 to April 29 2011.

ChartBeat's numbers are concurrent users. No idea how that translates on web but I'd expect it's more than 200m uniques - we do 120ish off 100k - 200k concurrents.

Absolute uniques per month is probably a good objective measure of size.

Don't think so. Chartbeat was affected by the AWS outage and he claims that his company own the server cluster.