• by Mark Walsh  @markfwal, November 7, 2011

There's little question that mobile is a growing platform for Facebook. The social networking giant now says it has more than 350 million active monthly mobile users across 475 wireless operators worldwide. As such, mobile has also become an increasing focus for the company, which rolled out a major upgrade to its mobile site earlier this year and an iPad app in October.

So how are people actually using Facebook on mobile devices, and how are they interacting with brand pages in particular? A new study by social media marketing firm Vitrue found that text and image-based posts on Facebook brand pages in mobile drew the highest engagement levels, drawing 21.5% and 20.4% of "Likes," respectively.

Following those two types of posts in performance were others including those featuring video, applications and Flash animation. On traditional site, by contrast, video, apps and Flash tend to generate higher interaction levels because of the larger screen and greater bandwidth.

Similarly, shorter is better when it comes to mobile Facebook posts. Across all page sizes, shorter posts -- those less than 70 characters -- received more Likes and comments than longer ones. Likes increased 4.3% on shorter posts, and comments, 31%.

Vitrue separated brand pages into those with more than 100,000 fans and those with more than 1 million fans. While Likes and comments are growing across both types of pages, those with 1 million fans are driving higher engagement rates. For example, large fan pages see a higher percentage of comments from mobile than less popular ones (4.51% vs. 3.23%). Mobile comments as a proportion of all comments on Facebook increased 57% from June to September, and Likes grew 22.4%.

Overall, mobile Likes as a proportion of all Facebook Likes increased from 18% to 24% during the three-month period while mobile comments rose from 9% to 20% of all comments.

When it comes to timing, the later part of the week and the weekend were the best times for brands to rack up Likes on mobile Facebook pages. Liking activity spiked on Thursday and Saturday and reached a low point on Tuesday. Friday sees the highest rate of comments. So Thursday through Sunday is the sweet spot for Facebook marketers in mobile.

Prior research has generally shown that both Facebook and mobile activity generally tend to ramp up on the weekend when people have more leisure time.

Looking at times of day, Liking in mobile spikes between 10 p.m. and 11 p.m. an 1 a.m. EST, with significant increases in the afternoon hours. Comments follow a similar pattern, with peak activity peaking at three specific times: 1 a.m., 6 p.m. to 7 p.m., and 9 p.m. to 10 p.m.

The Vitrue study analyzed user interactions with 1,000 randomly selected Facebook brand pages from June 1 to Sept. 30, 2011. Devices tracked included Internet-enabled phones, smartphones and tablets.