I like StumbleUpon. My website gets many visits from there. But as many ( blahblahtech and traffikd ) I have an issue with the friends limit.
Said briefly you can be fan of at most 200 people. After that you get a warning and you cannot add more fans. You cannot even reciprocate a fan of yours. So you are in a kind of deadlock.
I devise this sort of algorithm to deal with such a situation:

• Become fan of a friend
• if he reciprocates fine
• if he doesn’t, persuade him to unfriend inactive people he is fan of
• if he doesn’t reply for a while unfriend him, because he is not active or does not share any interest with you

In Actionscript :)

var limit:int = 2; // weeks

for each (stumbler:People in peopleYouAreFanOf) {
	if (!stumbler.reciprocates() ||
            !stumbler.persuaded() ||
            stumbler.getRepliesWithin(limit) == 0)
        {
		stumbler.unfriend();
	}
	else {
		you.sayThanks(stumbler);
		you.interactWith(stumbler);
	}
}

I started using stumbleupon. I like it! But I also like Safari. As you might know there is no Safari bar (just for firefox and the other well spread browser). I often have the need to stumble a url, to simply state that it is my favorite. So I created a bookmarklet. I added a new bookmark in my bookmark bar, whose link is:

javascript:location.href=’http://www.stumbleupon.com/submit?url=’+encodeURIComponent(location.href)

Any time I click the button I submit the url to my favorites. Nice uh?