From the steamworks page aimed at developers http://www.steampowered.com/steamworks/ (in the PDF brochure thing)
Steam is so awesome I'm willing to pay extra to get games on it rather than retail1100 games.
140 million Steam achievements unlocked.
1 million active Steam Community groups.
10 million Steam Community profiles.
25 million accounts.
2.7 million concurrent peak players.
500 million minutes played per day.
13 billion minutes played per month.
750 terabytes delivered daily during Holiday Sale.
200+ gigabytes/second of bandwidth.
20 petabytes of data delivered per month.