Rated 2 out of 5 stars
According to the add-on page for the StumbleUpon toolbar a mere 672,694 people. This makes SU's claim to have millions and millions of Stumblers a tad hollow.
This toolbar still does exactly what it was always designed to do (delivers web content according to preference) but what doesn't do any more is act as a gateway to a wealth of creativity and real social interaction that beat FB et al hands down. Download it and use it by all means but expect nothing more than a click "wow that's a cool (or crap) site" experience.