Wow. The number doesn't surprise me, but it's still a big milestone. I use Facebook on my iPhone and my Blackberry. I think this number will only continue to grow and everyone in the ecosystem will benefit. Reasons:

* More and more cell phones have larger screens/full key boards. The Facebook mobile site is good, but the applications are great. They didn't say how many of their active users are on a converged or more advanced handset like an iPhone or Blackberry, but I'd be surprised if the traffic from these devices isn't high. I tried to get Sam Altman to tell me yesterday – how Loopt was doing on the iPhone vs. a basic feature phone, but he wouldn't tell me. All he would tell me is "way higher."
* High level of overlap in the demographics of active data users on cell phones and those who use social networking sites online multiple times each day. They are one and the same.
* The more your friends post, the more reasons you have to check in more often and comment / respond. It's circular … and gaining momentum.

I've published some research on how important I think social networking is to the mobile industry. See summary here.

See Facebook post.