The number of active members can be roughly tracked by the number of uploads for each day - obviously there's no way of knowing how many members have posted more than 1 photo. So a few figures: Jan 1st 8598, Jan 15th 6609, Feb 1st 6107, Feb 15th 5121, March 1st 4167, March 5th 2971. So not all those who started on 1st Jan are still going I suspect!
@sburbidge that's not an accurate estmate. The original poster asked for how many members, not for how many "actively paricipating" members there are. You always have to assume there are members on here that have more than one acct. as well (would they count as one member or two?), members who have died and their families were not aware they had an account (do they still count?) and the members who are merely lazy (like myself) and do not choose to post every day but still a picture a day and post them all in one big lot (nothing in the guidelines saying we can't do that).