Flickr (which is arguably a dead man walking)
Flickr is still the best and largest community of amateur photographers on the web. It's not shiny anymore and Yahoo is letting it stagnate, but the community is still there nonetheless.
What I don't get is that people already upload tons of shitty photos on Facebook. What Facebook misses here is quality of the uploaded content. The signal to noise ratio is awful. That's exactly what Flickr has and Instagram doesn't.
That's why I don't get this acquisition. What does Instagram have that Facebook doesn't?
My first thought was the native mobile app, which is indeed cool. But I never used it, simply because my Android phone came with built-in integration with Facebook and Google's Picassa, making sharing easy. And I also need a centralized repository of photos from multiple sources, not just from my phone, which means I need lots of storage and good management tools. And if I want to retouch my photos, I like to do that from the comfort of a laptop or a tablet, because the display of a phone is too tiny, even for playing around with stupid effects.