Facebook's mobile users (via the Opera mini browser) grew by 600% in 2009; twitter's by 2800%

"Facebook is now the most-visited social network on the mobile Web, according to Opera’s State of the Mobile Web report. Unique users of Facebook grew more than 600% during 2009, helping the site surpass VKontakte, formerly the most popular social network among Opera Mini users. Twitter saw its usage increase more than any other social network, surging more than 2800% in just one year."
Source: Press Release from mobile web browser company Opera, 26th January 2010
Full report here