I swear Stumbleupon has some of the worst explained functions, I’m surprised there is not more help out there. It took me a while before I actually knew the send a page to a friend thing actually existed, then much scratching of the head to figure out where it was.
If you stumble a page and want the people who have friended you to check it out. Click the “Send to” button, it will then display all the people you can send the page to. Next time they stumble they get to see your page and a short message from you.
My tip is only send the requests to people who know will give it a chance, sending it out too much will lead to people revoking their friendship and you become a sad loser with no friends.