Facebook has all the tools in place for sharing things perfectly and privately just as I would want and I know I can even customize them per piece of media I put out there. The problem I have is working through 500+ people and figuring out who to share what with. I think the reason people are looking for an alternative for some aspects of sharing is that it's too much trouble to curate that giant list (and sometimes, as you mentioned, useful to just keep that giant list of friends). Instead of going through and curating all those people, it's far easier to go with a new service, proactively add just a few people you care to share with for any given event and then put your photos/content there.
I guess I understand that angle. Starting over is easy, but won't it be kind of a self-perpetuating cycle of social networks that we need to throw away and replace if we keep that up?
Would really love to hear if people agree, but am even more interested in why people might disagree.