Like yourself, the time I have for social networks is limited and yes, quality over quantity for me too.
For me, Google+, with its search and Communities, has been excellent at finding content and users that fit into what I define as a target audience. Of course, I am betting on the fact that having a solid presence over there will then help with my SEO efforts.
I am also active on a number of forums. As compared to Facebook and Twitter, it is so much more easier for me to find people who are passionate about a particular topic on forums.
Apart from these two, I will try a new social network every three months. Sometimes, you'll never know how a new platform may perform unless you spend enough time there. For the start of 2014, I'm looking at SlideShare but I'm not quite sure whether I'm too late to the party.
Duplicates aside (I don't think it matters on social media anyway), the way how users consume content and the types of content they prefer varies from one social network to another. Rewriting the headline, post or description will definitely help a link to stand out and garner more visits. For example, sharing a quote from a popular article instead of the title makes a tweet unique.
Also, I try not to blast the same content across all the social networks at the same time by filling the queue with different degrees of useful content created by others.