Stumbleupon has lots of people that dedicate time out of their lives to read interesting content on other peoples websites. if you want to build a following there, just add meaningful links and follow others who have links you find to be meaningful. The more you sharing and follow, the more likely you are to be shared and followed.
Like any other social media sites, stumbleupon is all about engagement. Meaning, in order to catch people's attention you must engage too. Like follow, comment, and most especially share interesting posts. IMO, those sites that are most likely to have large quality images on it catches my eye.
Creating an interesting profile can be helpful - add useful information, upload your photo and select specific topics you're interested in. You can also add a SubpleUpon badge to your website content
StumbleUpon works like other social media sites. To catch attention of other users, like posts, comments and follow other users. Engaging with other users is the key to getting more followers.
Do very many people still use StumbleUpon? I honestly thought that ship had sailed a few years ago. Perhaps not?
In my opinion this is not one of the most popular social media sites, like it was several years ago. But on the whole there is no something special in its operation, interact with your followers, like share and post valuable content. Such way will help you to promote your page.