Slashdot's ten millionth comment sums up the whole thing very well.
- Post 1M on Jun 15, ????
- Post 2M on Mar 1, ????
- Post 3M on Feb 13, ????
- Post 4M on Aug 2, 2001
- Post 5M on Jan 2, 2002
- Post 6M on May 20, 2003
- Post 7M on Sep 18, 2003
- Post 8M on Jan 16, 2004
- Post 9M on Apr 28, 2004
- Post 10M on Aug 18, 2004
- Post 4M to 5M: 153 days
- 5M to 6M: 138 days
- 6M to 7M: 121 days
- 7M to 8M: 120 days
- 8M to 9M: 102 days
- 9M to 10M: 112 days
At a roughly constant rate of posting, it will take 3 years to post 10 million more comments, or about 27 years to get to 100 million.