I agree with you that privacy is very important, we have a lot of stuff that we don't share online specifically because we value our privacy.<p>My point isn't so much that you are doomed, you need to sell the general public (such as myself) on the idea that your system is unique and valuable. You need to figure out why private, intimate sharing is really important and totally sell the benefits and the ease of use.<p>You also might want to consider slowly regenerating credits, say every day you get 1 free credit to a limit of 30 accumulated free credits. That way noone will leave your site for good after putting 30 photos online and missing the point. Send an email when the points max out to remind your users to come online (habit forming?)<p>Perhaps you could have album content invitations - a user asks another user to add a certain type of content - "could you upload your birthday photos for Alex please?" That is a nice excuse for user A to remind user B to log in.