[another]

But what sort of traffic?

See, e.g., Arvind's comments on StumbleUpon visitors:

http://arvindn.livejournal.com/133249.html

To quote the takeaway, "most of the traffic generated by StumbleUpon users to any given site is going to be low quality because the dopamine junkies make 100x more clicks".

[phirephly]

As far as I can tell, SU traffic is almost useless. I may get 1,000x as hits from SU, but I derive more value from some hole-in-the-wall site writing about me and sending 20 people.

[muyyatin]

Very true. StumbleUpon shows a much higher bounce rate and much lower average time on our site.