Interesting piece on ReadWriteWeb about StumbleUpon which is another of those smaller social media success stories that has been around (founded way back in 2001) and continues to quietly grow, but who exactly uses it?

StumbleUpon: The Silent Social Media Success gives the story to date, which is this: StumbleUpon now has 10.6 million users and continues to push traffic to publishers.

But unlike say the better known, or more talked about, Digg and Slashdot, it is hardly talked about despite some sizeable numbers.

ReadWriteWeb quotes analysis by real-time web analytics firm Woopra that says StumbleUpon drives nearly twice as much traffic as Digg and is apparently second only to Facebook among social media traffic drivers.

But then when you look at the most popular content on this crowdsourced social media site it is an odd collection. Lists are a big feature as is humour and take a look at some of the content on the top rated list:

1. Weird and twisted on the Behance Network

2. Scarlett Johansson by Sheryl Nields – Touchpuppet

3. The Decade in Pictures – Pixcetera Blog

4. Bad ass LEGO Tattoo – Pilot Extra-fine | Ufunk.net

5. In Persona – What’s in your bag? | Ubersuper

Last year the top rated content included such perennial winners as ’99 Things You Should Have Seen On The Internet’ (471K Stumbles) and  ‘Life Summarized in 4 Bottles’ (439K Stumbles) and the like. Much of this sounds like the kind of material that content farms are aiming to produce — so really the question is how valuable/useful is StumbleUpon?