Thanks to a new post-logout mobile promotion by Facebook, the company has seen a jump in overall mobile usage in the past week, most significantly on the iPhone where mobile usage has grown 20 percent. While there could be other reasons for the jump, we’ve noticed that the growth in iPhone usage has coincided with a less significant jump among Blackberry users.

The latest data follows new statistics from comScore earlier this week which showed a 30 percent increase in social networking traffic among smartphone users over the past year. There has also been increasing discussion about the shift away from desktop computers toward mobile platforms. The past week’s jump in traffic however, illustrates Facebook’s ongoing embrace of the shift toward mobile devices, and their efforts to push users in that direction.

Facebook also continues to expand it’s user base as it has exploded past 400 million and appears to have continued growth. As Mark Zuckerberg has emphasized repeatedly, most of Facebook’s interaction will take place off of the website Facebook.com in the future. Whether that can be interpreted as part of the Facebook Connect strategy or simply that it’s part of the shift toward mobile is unknown.

However Facebook is clearly dedicated on the mobile growth and these latest statistics highlight the important of mobile in Facebook’s future growth strategy.