The Fitbit Flex is a pedometer, a device that counts your steps. (It also can monitor your sleep but that is beside the issue here.) To calculate how many calories you have burned Fitbit uses the information it has about you at that current moment: age, weight and height to determine how many calories will be used by taking one step.
However, various activities such as running, jumping, dancing and all other activities will burn different amounts of calories because they will use more muscles, increase your heart rate, and so on. Fitbit itself however cannot monitor this. It just counts your steps. However, other things can monitor that if you want and some of those have intigrated their service with Fitbit.
Just as others have said to have abetter idea to know how many calories you have burned during anything else, but taking steps you would need to take a lot more into account such as heart rates and the likes. There are plenty of devices out there and even more applications that can help you monitor and share that information.
If you are going to use another service that can connect with your Fitbit account it will do the following: it will send their data to Fitbit. Fitbit than just puts that data as an activity in and use the calory value for that specific time span given by the service assuming it is more accurate than their projection (and it is more accurate). By doing things like that Fitbit will be far more accurate in projecting the amounts of calories you have burned during various activities Fitbit cannot monitor on its own.
Fitbit is not an all in one solution, but by intigrating Fitbit with other services to pick up where it cannot be the best, it will become a place to go to monitor most things you want.
I hope this has helped.
Follow my adventure from couch potato to decent runner: http://fitnessnick.wordpress.com/
