How often do you use stumbleupon?

[cocaine]

I have 3 sites.
I am testing stumbleupon since three months and the good thing is I am getting 20% of my daily visitors from stumbleupon. It is really really easy way to get traffic. I recommend everyone to use this. You can do exchange stumbleupon bookmarking very easily using stumbleXchange :: Exchange Stumbles to Promote Your Website you can get people to exchange stumbleupon bookmarks in this website.

[Archbob]

I read somethere that stumbleupon uses the <nofollow> tag now so that google will not spider it. If this is the case, then stumbling is alot less valueable than it used to be. Doesn't stumble upon take any measures to protect themselves from people doing promotion exchanges?