TripAdvisor has 45m reviews & opinions, and over 6m photos

"Milestones in the history of TripAdvisor:
- November 2000: http://www.tripadvisor.com goes live
- March 2002: The site is profitable
- April 2004: Sold to IAC
- August 2005: Part of Expedia, Inc.
- January 2005: The first hurdle of a million reviews and opinions is achieved.
- April 2008: 15 million reviews and opinions
- July 2009: 25 million reviews and opinions
- May 2010: 35 million reviews and opinions
- August 2010: TripAdvisor becomes the first travel site to have more than 40 million monthly users* to become the world's largest travel site.
- October 2010: 40 million reviews and opinions
- 2010: TripAdvisor launches 10 new sites around the world
- March 2011: 45 million reviews and opinions covering:
1) 85,000+ destinations
2) 474,000+ hotels
3) 100,000+ vacation rentals
4) 135,000+ attractions
5) 675,000+ restaurants
6) With 6,000,000+ candid traveller photos"
Source:  Press release from TripAdvisor, 12th April 2011
Note:  I'm not sure what constitutes an 'opinion' but I suspect it includes all comments on the forum