Very interesting that they choose StumbleUpon as one of the least popular social channels with search marketers - particularly when juxtaposed with this infographic:
http://columnfivemedia.com/work-items/t ... umbleupon/ (Also, this site has a bunch of great infographics - check them out! Particularly team members involved in working on our own infographics.)
To some extent I can understand why StumbleUpon isn't popular with search marketers, in that (without using their paid service) it can be difficult to promote your own content in a non-spammy way effectively. The traffic sent by StumbleUpon, while likely engaging with the promoted content well, still ends up being very high bounce traffic due to the nature of the service. Most stumblers will continue stumbling instead of exploring the site they landed on more unless there is a very strong call to action that catches their attention. (I can speak to this both from experience marketing through SU and as an avid stumbler myself.)