eBay-owned StumbleUpon is making efforts to improve its availability beyond Firefox users that have registered and downloaded its toolbar.

The service enables idle surfers to "Stumble" through random sites over the 'net. (Select sites are indexed, but publishers can also add pages to the StumbleUpon network.) As users flip through them, each page can be granted a thumbs-up or thumbs-down, which affects future sites they see.

These "gut" preferences are combined with user data, like age, gender and location, that enable StumbleUpon to target advertising to them. Ads are typically sponsored webpages that look almost like any other Stumbled site. (They can also be rated.) Part of their innocuousness is how rarely they appear: for every 100 Stumbles, less than 10 are sponsor pages.

Moving forward, users no longer have to register to use StumbleUpon. In fact, they don't even need Firefox. (StumbleUpon began its life as a Firefox add-on.) Anyone who visits the StumbleUpon website can now Stumble, rate and save favorite sites.

The social media company also began a partner program with four major web publishers: Huffington Post, HowStuffWorks, Rolling Stone and National Geographic. The sites will enable StumbleUpon users to Stumble through their content without having to register for access.

But while registration is no longer necessary to using the service, it remains a prerogative. "This is about giving users the experience of 'Stumbling' and then getting them to register," said GM Michael Buhr (via MediaPost).

eBay acquired StumbleUpon last year, due in part to its rapid growth. The userbase more than doubled in the past year to about six million people, generating over 350 million Stumbles per month.

Growth since then has slowed. Traffic to the site fell to 675,000 as of August, according to comScore. Last month, insiders reported eBay plans to sell the site.