An SF Chronicle article earlier this week pointed to an internal Google document that forecasted 372K new advertisers over the next four years (at a fairly even ramp of 90-100K/yr).
These seem like low numbers to me, especially given that (as far as we know) it includes a) International growth, b) “Local” growth (and ‘local’ outside of the U.S. domestic market) and c) future advertising offerings (which might attract a new breed of advertisers).
For now, I’ll chalk it up to conservative forecasting, misinformation and/or consolidation of accounts through reseller/channel partners (SEMs, IYPs, etc).