Facebook's role in marketing has become hotly debated. Both critics and the company itself tout studies that address whether advertising on the social network is effective or not. Chalk two more up for the critics. According to one, the shelf life of marketing is only about the length of a very long day at work. The other says that Facebook's online U.S. ad market share is dropping.

Shelf Life

Global communications agency OMD, part of Omnicom Group, found that the average shelf life--the time over which people interact with content--of Facebook posts is about 18 hours:

"That was the biggest surprise," said [U.S. director of OMD Word Colin] Sutton, referring to the short feedback cycle for most marketer content posted on Facebook and analyzed by the study. It analyzed nearly 300 content posts across the pages of 10 TV networks over a four-month period, November 2011 through February of this year.

The focus on interaction--such as, leaving a comment--is narrow but it's one of the few ways that an analysis can tell if someone is engaged with the content absent some more complicated study that could match exposure with after-the-fact actions, as comScore did in its latest study of Facebook with conclusions that were somewhat questionable in the context of the actual data.

Shelf-Life Boosters

OMD only reported on one industry category, certainly, but Sutton said that the "general findings still hold" in other verticals. Thursdays and Fridays seemed best for user engagement, while Sundays and Tuesdays were the worst, though no details by how much. A photo can boost the shelf life by 9% while a video can stretch it by 16% on average. Still, that would only stretch things from 18 to just under 20 hours.

To put that into a greater context, the shelf life of content on YouTube can be measured in weeks, or even months, if content goes viral.

Mobile May Pay Off More

In the second study, IDC published its latest online ad data. Here's something from IDC program vice-president of media and entertainment Karsten Weide:

Facebook's domestic advertising growth has slowed dramatically, from growth rates in the 60s% in the first three quarters of 2011 to 30% in 4Q11 and now to just 21.2% in 1Q12. This has led to an actual market share loss after what seemed to be an inexorable rise, from 13.9% of U.S. display advertising in 4Q11 to just 12.0% in 1Q12.

The Facebook market share decline is happening against a backdrop of an 18.7% increase in U.S. online ad spending between 2011 Q1 and 2012 Q1.

In addition, mobile advertising market share is way up, growing from 2.1% to 9.8% in the same time period. Search ad share, though, was down from 54% to 42.4%, even though the category as a whole grew by 8.8%. The big driver here is the shift to mobile ads from traditional PC-based.

The question for entrepreneurs will be whether mobile really pulls its weight. If the results suggest that it does, expect that share of online ads to jump significantly--and for Facebook to face increasing pressure to monetize its mobile activity.