Facebook has designed their Skype-based "Video Calling" feature specifically for each browser and OS combination out in the market. Unfortunately, you might notice if you are running a pre-release version of Mac OS X Lion that it simply doesn't work. In reality the Facebook video calling feature will work on Lion but is blocked due to the unofficial nature of Mac OS X Lion.
The fix is easy however: Just change your Browser user agent to something a little more traditional on any modern browser in Mac OS X Lion and you are video chatting.
For instance, in Safari, hit up Safari 5.1-Mac and you are golden.
Google Chrome: When Chrome is closed type the following in terminal:
open /Applications/Google\ Chrome.app –args -user-agent="Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10_6_8; en-us) AppleWebKit/525.19 (KHTML, like Gecko) Version/3.2.1 Safari/525.19"
Or, you could download this Chrome plugin that'll allow you to switch the user-agent directly within the browser.
Firefox: Simply download this plugin and select the user-agent as shown in the image below:
Once you have modified the user-agent you may also need to enable Facebook Video Calling for the first time. To do this simply navigate to http://facebook.com/videocalling and click get started. You're all done.