I mean actively participate in, not are a member of.

Two. Facebook and Twitter. But Facebook only in a distant, there's-a-distinct-taste-of-bile-in-my-mouth way. I comment, but I keep my posts as far from profitable for Facebook as possible. But Twitter, I love.

Oh, and Instagram, if it's made the jump from Photosharing service to social network yet.

I spend in total approx 15 minutes a week on Pinterest and FB, and maybe only 2 mins on the latter.
On Pinterest I try to upload contents (TED vids/docos etc) of some value to people who follow me, but I personally haven't found it useful for myself. Either I haven't used it long enough or I am not a pregnant woman.
I only use FB to share photos and organise events, other than that it's just another form of procrastination to me.

I actually want to ask what are the features/experiences you really like about social networks and what are the things you dislike about them? Basically what makes you want to use some, and what makes you want to steer away from some? The more specific the better.

Twitter. That's it. Used to be all about Facebook, but no more. Have an account only to help manage some pages, but that's it. Have an Instagram account, but barely check or post to it.
I use Twitter for professional development by developing a PLN and having many conversations with fellow educators.
I like Twitter because it's simple, succinct. 140 gets you to the heart of the matter. It's easy to follow others' feeds.

I started SM on FB in early days, when it was a more sophisticated alternative to MySpace. It became a perpetual high school, old school, this-is-your-life reunion. I tired of this; did not want to reestablish and maintain so many old connections. We weren't really friends. It felt inauthentic.
Words have power, and the word friend means something to me.
I've since reopened two new Facebook accounts, wanting each time to use it with true friends & family to authentically share photos & updates. My latest account is purely to help manage some Pages.
I don't trust Facebook, don't like their Internet usage trackers, don't like their automatic access to my address book, don't like them tracking purchasing through credit associated w/ my email.

I use 4 regularly, in this order: Twitter, Google+, Facebook and Linkedin. Facebook has become a business account so I use it to network with friends in the business more than family. Twitter is my main channel of communication and for leads for my business as well.