Facebook Inc (NASDAQ:FB)’s efforts to begin stealing ad spend from television have been well-documented by analysts who follow the social network. The company said it will use data from Nielsen to measure how well its new video ads will do, which most expect to be a good think for it. However, a major discrepancy between Nielsen and comScore data calls into question the accuracy of that data.

How much TV time?

Pivotal Research analyst Brian Wieser pointed out the discrepancy in a report dated March 21, 2014. He notes that being able to measure consumption of online video is an important part of how strong online video. Ultimately, consumption could be a key driver in how much Facebook Inc (NASDAQ:FB) can charge for its video ads. However, the social network might be selling itself short by using data from Nielsen instead of comScore.

As this graph from Pivotal shows, online video content makes up either 3% of TV viewing time or 9%, depending on whether you’re looking at Nielsen or comScore:

Facebook online video

Facebook votes for Nielsen

Wieser believes both sets of data are wrong because they exclude video from tablets. However, he believes the real number is closer to Nielsen’s data, which is about a third of comScore’s estimates. The fact that Facebook Inc (NASDAQ:FB) is going with Nielsen shows that it also has more confidence in Nielsen’s estimates, which says a lot since they are so much less than comScore’s.

Of course most professionals from media agencies prefer comScore’s numbers because they are higher. They also prefer comScore because in general, its products are seen as more accurately measuring online audience share. Share is the critical component when it comes to deciding which media publishers should be on a request for proposal on a particular media buy.

However, Wieser points out that comScore and Nielsen have different methodologies, which could become more important as online advertising becomes more and more important to big brands.

Will Facebook take TV ad spend?

The analyst believes that any efforts by Facebook Inc (NASDAQ:FB) and other online advertisers to actually steal part of the TV advertising pie will be in vain. Nonetheless, he thinks that the new online ad units still will enable the social network to expand and, in the meantime, promote Nielsen’s measurement capabilities to digital media buyers who otherwise might have focused on comScore data.