Don't think so. Your hypothetical app saves all of my photos, but I suspect people spend far more time browsing other peoples' photos. Those I don't get by being the first to migrate to a Facebook competitor. That's where the powerful lock-in lies.
no it doesnt. when something "better" shows up people will want to move on, but you are right there will be like "oh, all my photos and tags, I cant leave them". "But wait, there is that App that Joe programmed and Mary, Mike, John and Jennie already used it, I can use it too and all my photos will be re-linked and re-tagged on that new site. yay me!"
