For some general statistics about AO3, you can check out:

• Our past news posts about site stats
• The OTW 16th Anniversary Survey Report which contains data from a survey of AO3 and other OTW project users
• The 2012 OTW Community Survey Report which contains data from a survey of OTW community members conducted

You can also find infographics with stats relating to all our projects in our Annual Reports.