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StumbleUpon is hot.

The discovery and recommendation engine that makes web browsing a lot like channel surfing just announced it’s now handling 1 billion stumbles per month.

On top of that impressive number, StumbleUpon just closed a second round of funding in March, wrapping up $17 million of series B financing.

That 1 million stumbles-per-month statistic represents explosive growth, especially when you consider that since just a month ago, that number has grown by 200 million, judging from the fact that StumbleUpon publicized 800 million stumbles just last month, according to Business Insider…

Why is StumbleUpon growing so fast? In my opinion, because it’s fun. Moments of serendipity run rampant as you click the Stumble button to go to the next selected site, and every site you see is picked because of the detailed preferences you indicate in your profile. It gets even smarter about your preferences as you vote for sites by clicking either a thumbs-up or thumbs-down icon. And, because I’ve been using the service for the past six years, it’s gotten to know my preferences quite well, and its ability to predict what sites I’ll like has become positively uncanny.

In addition, it’s hard to tell you’re being advertised to, because StumbleUpon uses your same preferences to determine which paid sites to show you. I think any business model that can figure out how to advertise to people without them being aware of it has a great chance of survival.