Ad traffic on  Facebook is being significantly undercounted by comScore and other hit recorders because the social site keep coming up with new ad formats.

comScore and Facebook have talked about the technical discrepancy and the hit counting firm is working on a fix.

Counting ad hits on viral sites that frequently create new forms of ads that blur the lines between advertising and content can be an accounting nightmare particularly when the changes occur in the middle of the scoring period.

comScore counts display ads according to the definition set by the Interactive Advertising Bureau, an organization that sets advertising industry standards and conducts online advertising research.

But advertising formats on social networks can be a fast-moving target.

“If you are just looking at display ads you are not getting the full picture,” said Debra Williamson, an analyst at eMarketer. “But Facebook and comScore are working together to correct the problem.”

Facebook hits are being undercounted by a factor of five, a source who requested anonymity said, so instead of the 1.1 percent market share it currently owns, according to comScore, the social-networking site should be scored at 5.5 percent share.

In the business of ad metrics, where spending decisions are made on percentage points, that is a significant discrepancy.

The ads that were not being counted were “engagement” or call-to-action ads that require site visitors to do something such as pick which movie is going to win an Academy Award.