I spend quite a bit of time on Google+ which is as much a community site as it is a social media one. I tend to participate in the areas related to my work and hubs, such as small business, entrepreneurialism and productivity. Because I'm self employed, I count this as work and marketing, rather than (just) frivolity and time wasting!
I don't tend to go on Twitter much as it's a blizzard of 140 character updates and Facebook I probably spend about 20 minutes on a day. The 'socialmedia' site where I spend most time? Why, Hub Pages of course!