More broadly, the click-through rate on wall posts on marketers' Facebook pages was highest from Monday through Wednesday, averaging 9.72% before dropping sharply to 4.39% on Thursday, and dropping further to 2.67% and 2.7% on Friday and Saturday. On Sunday, the rate starts rebounding, at 7.63%.

The findings follow ViTrue's initial release last month of click-through data for brands' Facebook wall posts, which average 6.76% and stand in stark contrast to the anemic rates for display advertising generally on Facebook and other social networks. The company comes up with its estimate by assuming that about one-twelfth of the total Facebook audience is online at any given time, available to see a message.

The number of fans a brand has is also factored into the calculation, with the total number of clicks on a specific post divided by the number of fans who saw it, adjusted to reflect that not every fan is on Facebook all the time. So if a brand has 100 fans and a wall post gets five clicks, that's a 5% click-through rate. But if only 20% actually saw the post, it would be a 20% rate. It comes down to a mixture of math and educated guesswork.

The latest research goes a step further by suggesting that certain days of the week might be better for launching a promotion or other marketing effort on a Facebook brand page. "This highlights the importance for marketers of having a well thought-out communication strategy for updating their brand page and when to create wall posts to get the optimum response," said ViTrue CEO Reggie Bradford.

He surmises that response rates are generally higher early in the week when people are more active on Facebook fan pages, and drop off as users turn to other pursuits on the weekend. Rates start to ramp up again on Sunday as people begin planning the week ahead via Facebook.

"The point isn't to say only do wall posts on Tuesdays," said Bradford. "The point is that there are differences among days of the week and thinking has to happen behind the wall posts so you can maximize the impact."

Shiv Singh, vice president and global social media lead at Razorfish, agreed that the timing of new messages on Facebook pages is an important consideration for marketers. "So the question ViTrue is trying to answer is a very valuable one." But he added that interaction can vary among specific pages depending on a variety of factors including the type of content and audience demographics, making it hard to generalize about the timing of new posts.

He added that he'd have to dig deeper into ViTrue's research and methodology to gain more assurance of its validity.

In the coming months, ViTrue plans to refine its research further by releasing an analysis of click-throughs on wall posts by daypart. ViTrue's findings are derived from its Social Relationship Manager suite of planning and reporting tools, which it launched last month. The sample data is compiled from wall posts garnering nearly 200 million fan impressions from March through August 2009.

ViTrue does not disclose how many Facebook brand pages were included in the study, but says it encompasses several Fortune 500 brands across several categories.

"As tools and platforms become more sophisticated, we'll see more segmenting of messaging by daypart and all those other types of measurement," said Bradford.