Of all the sites I looked at, Reddit had the most interesting articles. First of all there were more to choose from and secondly, they had more of a left-leaning slant. I read a few articles on each site, but the most compelling was on Reddit.
At this point, they detract from my productivity. If I am busy reading what the latest buzz on the web is, I'm less inclined to help my patrons or organize my books, make displays, plan activities etc. I know of one person who uses the StumbleUpon site and he is notoriously unproductive. I hate to be cynical. There are some great articles on these sites, but they don't relate directly to what I do.
I read a great article on the Reddit site (can't remember where it came from) that was about the 7 characteristics of a country about to undergo a revolution. The author successfully argued that the U.S. had all of the qualities of a country on the verge a major change. Heady stuff. Useful? Not sure.