Over the past few months, there has been a fascinating development in how people are being referred to Web sites: StumbleUpon has emerged as the most popular traffic sources – something that popular blogger Fred Wilson (aka a VC) talked about yesterday in noting his traffic had doubled recently.

After a long period of being ignored while it was part of the eBay empire, StumbleUpon had jumped back into the spotlight as a more popular traffic driver than Twitter and Facebook.

While StumbleUpon doesn’t consistently deliver traffic to my personal blog, it does appear on a semi-regular basis, which is always pleasantly surprising.

But here’s the thing about StumbleUpon: what is the quality of the traffic that it drives to Web sites? As SocialMouths made abundantly clear in a recent post, StumbleUpon is a “content discovery tool” that people use to browse multiple Web sites based on their interests.

As a result, it is not really a service used by people to find specific Web sites. It means people are “tasting” rather than “eating”. So rather than doing a search and then spending a fair amount of time on a Web site, many StumbleUpon users stumble upon a Web site before quickly clicking to the next one.

So before Web sites get too excited about StumbleUpon has a traffic driver, they need to ask themselves whether these visitors have any value.

Are these people, for example, going to look at and act upon advertising, will they ask for more information about a product or service, or will they purchase something?

It would be fair to suggest some of them might turn into customers but I would suggest the vast majority are taking a quick look before moving on to the next Web site.

This isn’t to suggest StumbleUpon has no value because any source of new traffic is good thing. But before anyone gets too carried away with StumbleUpon, they should consider the type of people that use it, and how much value they have.