Foursquare/Swarm
People do things at places. If you don't write a status message in your checkin (most don't), I can still sense what you might be doing. I try to write down what I'm doing at the places I check in at, mostly for historical interest later on. It's what Twitter should have been and was for a while, the “What Are You Doing?” social network.
Reading.am
This social network tells me what people are reading (and watching and listening to, if the website has the right content type associated to it in the metadata).
Bandcamp
You can follow bands and there's a feed. And you can buy their album for a decent price. SoundCloud would be here if they knew how to make money.
Instagram (though I wish Flickr had wised up to it early enough, since Flickr does so many things right on a technical level that Instagram just won't do).
The best place for the news. If you see someone saying “Why aren’t any of the newspapers covering [x]?” then stop looking to newspapers to spread the news. The news is spread on Twitter. I have real problems with Twitter as a medium and I think the format influences the type of content there, that is, it's not a problem of who I'm following but rather the incentives are for spreading memes, making jokes, overwhelming with news, not as a medium for calm reflection. Their API is the best in the business (Swarm's is actually slightly better, if less well-known).
Only to look at my cousins' baby photos. I still don't know how to Facebook. I use it to signal my interest in events. People that put on events: list your event on Facebook.