Facebook: your mom likes your status, because you know your mom is on facebook, so you can't post anything real there.
Twitter: Unless you're a comedian, you're going to fall into one of the three categories of useless tweeters: the vapid, the incendiary, and the guy who literally just links to his own youtube videos.
Instagram: Unless you're a photographer or some shit, nobody actually takes enough interesting pictures to maintain a presence. So you're probably posting selfies, pictures of your pets, pictures of your food, or you're a pedant wannabe photographer who takes pictures of the stopsign outside your hose with a black and white filter on.
tumblr: Nobody actually puts anything on tumblr. They just reblog things that were already on tumblr. Also, it's shockingly difficult to have a meaningful conversation on Tumblr, since you can't message people back and forth. All you can do is reblog things with a comment. Also, obligatory bitching about SJWs.
Whatever I'm missing because I'm too old to know about what the newfangled social networks are these days: sucks because it's full of children.
Google+: PEOPLE ACTUALLY TALKING ABOUT SHIT. Like, for really real. It's basically just a web forum like this one, updated for the age of social networks. With circles you can control who gets to see your content, and with collections you can separate your content into groups for people to either follow, or not.
Here's the big downside, and the reason everybody thinks google+ is a ghost town: you've gotta actually have something to talk about. If you're just a vapid 20-something who wants to rack up the Internet Points for posting pictures of "feeling sexy today," then you're not going to find a bunch of other vapid 20-somethings to talk to. BUT, if you're the kind of interesting person who actually has a passion in life, you can find people to have a conversation with about that passion.