You can now add TweetDeck to the growing army of apps that are joining Facebook Connect. TweetDeck is the most popular desktop client for Twitter, and now it will add Facebook functionality as well. When you send out a message from TweetDeck, you will now have the option to send it to Twitter, Facebook, or both. A new column will be added to TweetDeck showing you status updates from your Facebook friends (in addition to the existing columns for Tweets from people you follow on Twitter, replies, and direct messages). And if one of your Facebook friends is online, a green dot appears by their name and you can chat with them through Facebook chat via a pop-up window.

TweetDeck will be rolling out the new feature over the next few days. A version of the update will be posted here for those who want to upgrade manually.

TweetDeck is just the latest app to join the Facebook Connect Army. On Saturday, Facebook announced that Facebook Connect now works with iPhone apps and desktop apps such as Seesmic. With Tweetdeck, Facebook ventures into the realm of Twitter apps. By going after the Twitter apps, Facebook can do an end-run around Twitter. Tweetdeck alone accounts of about 13 percent of all Twitter users.

If more of the top Twitter apps add Facebook Connect, more than a third of all Twitter users could be covered pretty quickly. Of course, Twitter could just add a Facebook Connect option itself, but that would be conceding victory to Facebook.