StumbleUpon’s share of traffic sent to publisher sites is down 53% since July, according to Shareaholic, which released some new data for October.

Update: StumbleUpon says its internal data is quite contrary to Shareaholic’s findings.

The trend might be in some part due to the rising popularity of Pinterest, which continues to grow its share.

“Additionally, Twitter now outpaces Stumbleupon’s share of traffic by .22%,” says Shareaholic’s Janet Aronica. “StumbleUpon recently unveiled a new site design and Pinterest-like ‘lists’ feature which enables content consumers to curate collections of articles they enjoy – but publishers will still have to see if this results in click-throughs and traffic directly to their sites.”

“If you’re monetizing through ads and sponsorships, you know that pageviews matter,” adds Aronica. “Any time you’re considering traffic from StumbleUpon, though, you should remember that everything is wrapped in an iFrame, so people may be clicking through to read your articles, but they may not be clicking through directly to your site. Therefore those pageviews aren’t recorded as referral traffic. As with all content creation, remember that the best-in-class, most compelling content truly wins. If you peak someone’s interest at first stumble, they’ll likely find a way to explore your site some more. For example, they may copy/paste your URL into the address bar so they can see more of your content, in which case they would be counted as direct traffic – not referral traffic from StumbleUpon.”

You can read what StumbleUpon had to say about the iFrames earlier this year here.

Meanwhile, it looks like the company is looking to send some traffic back to its own site, as it has removed the Explore Box from the toolbar, making it so that users have to go to StumbleUpon.com to access the feature, which is essentially the StumbleUpon equivalent of search.

We’ve reached out to StumbleUpon for comment on the traffic decline, and will update accordingly.