The NSA has been in the IT security game for a very long time, they employ the best of the best, and have practically unlimited funds. I'd imagine that very complicated algorithms determine who to monitor and what keywords to look for. Images from the middle east or a VPN are likely more heavily analyzed than images from a college campus inside the US.
Why set up shop at specific social media companies when they have physical access to backbone routers and root certificate private keys?
Yes, it would be easier to just ask FB/Google/Apple to give them unlimited read access to their databases, but that would be a scandal waiting to happen.