usually it ends in a while.... a short burst and then.... then you stick with your real web site visitor numbers
SU is usually spikes, but if you keep producing quality content, you can get new stumbles all the time.
You get high numbers because you partcipated in a lot of stumble exchanges. They dry up as you quit doing exchanges. Also be aware that Stumble Upon is starting to ban people like digg does if you are caught doing exchanges.
If stumblers like your content, then you'll get natural stumbles from stumblers. There's nothing wrong with that.
It's real traffic. The problems with StumbleUpon: Although it's semi-targetted traffic, these are flighty people, if they don't like your page immediately they'll click Stumble! again and move on. If your page is crap, such as an MFA (Made for Adsense) type of page, you could even get reported to StumbleUpon for abuse. This could lead to blockage of your site from being stumbled and/or your StumbleUpon account suspended. StumbleUpon should NOT be abused, as they are coming down on people, like Digg did in the past.
Stumble Upon is a pretty great service. Check it out if you have some time! It'll explain everything about what they do and how they do it.
There is very low conversion with Stumble Upon traffic. It is real traffic but often times they just stumble for fun. Try it for free before putting out the money to pay for stumbles. If you are not signed up, get an account and stumble your own sites. You can participate in some exchanges on here but be careful because you may end up with a blocked site or a banned account.
i just joined myself. i'll see if my sites get stumbled? did you just add your site and that's it? nothing else?
Take a gander at this It'll explain a lot. Basically, the more stumblers positively stumble a page, the more it'll show up for people to stumble. The more fans those stumblers have, the more it'll influence the page showing up to be stumbled on StumbleUpon. The more reviews your page gets, the more influence. The more people that ARE NOT part of your fan network that positively stumble the page, the more influence.
It will only keep up if your site is viral/sticky/addictive. It normally trickles of, but glad you got some good traffic from them.