Flickr is still big in numbers but has more or less gone south for a lot of users, and Instagram may have filled this gap.

there's a lot of people that wouldn't fit the bill for facebook friending but are perfect in a looser setting. In this perspective, even if userbase has overlap, I'd guess that the social graph on Instagram is radically different than the one on facebook for a lot of users.

Now depending on how much Instagram gets integrated into fb, that might become less and less the case and give an opening to another photo sharing service...