I will allow as to how you may be right, Stainless -- but how often has it happened here? Not many times, and I do look at each and every post. And how often has it happened to photos I put up there? Dang near never, as I do keep my paid membership up to date.
I've had a few disappear when the Photobucket naughty-photo machine decided that you shouldn't (for instance) see THAT photo of Nancy's tattoo. But by and large it hasn't been an issue as far as I know.
But in theory you've hit on something valid. I don't put photos into the Photogallery vie Photobucket, for instance, because the gallery demands permanence. But also -- we don't have any size or quantity limits there. The Forum software does have limits.
As for other photo-sharing sites (Flickr, Kodak, etc) -- I don't have much experience so can't recommend or decry them, other than I do know that Kodak requires some signing-up to do more than merely view them (my daughter puts the baby pictures there).
So I'll continue to use PB and will continue to be careful about shots that I insist must remain for the long term. If there's a way around the Forum's limitations -- I don't know of it. Maybe Jon Amo could figure out one -- and if he does (you reading this, little A?) I hope he'll share it with us.