I once thought StumbleUpon was completely nofollow, but then I realized that it isn’t completely so. When you bookmark your web pages at StumbleUpon, those pages are nofollow. But your profile links are not. What that means is you can add links to your profile at StumbleUpon and those links are dofollow, completely.
There are many social bookmarking sites set up that way. StumbleUpon is not the only one. The reason links in your profiles are dofollow and all others are nofollow is because these sites don’t want you to show up and spam them thinking that you are going to build links back to your website. That isn’t the case and there a couple of reasons why.
No. 1, Google only counts so many links from one site. Yahoo!, on the other hand, doesn’t but that’s really a side issue. Those links will still count as inbound links to your website, but you have to build a lot of them.
The second reason why StumbleUpon, Twitter, and many other social media sites pass link juice through your profile and not through the actual bookmarks is because they really want to reward legitimate users for using their site. If they didn’t give you link juice in your profile then it would all be completely social and there would be no search engine benefits at all to using the sites. But they don’t really want that. They want you to be rewarded with higher search engine presence, but they don’t want to be spammed to death so offering link credit from your profile page but not from the actual bookmarks is a fair compromise for you and for StumbleUpon.
So how many links can you have in your profile? I’m not sure there is a limit. But there is a limit to how long your profile can be. And as long as you aren’t using it to be a spammer – that is, you are putting legitimate websites that you own in your profile and a fair amount – then StumbleUpon is happy (and so is Google).