I don't use a Fitbit, but what I have seen in MFP is that is "double counts" activities. For example, on days that I run, the MapMyRun app tells the MFP app that I ran 3 miles and burned 350 calories. Then MFP realizes that I took 8000 steps that day and gives me a calorie adjustment (say 200) for that, BUT it is counting as part of that the 4000 steps that were part of my run. So it double counts the calories running steps. So it gives me a 550 calorie adjustment (350 + 200) vs the 450 (350 + 100) it should really be.
With MFP I just try not to eat any of the calories that I have "earned back" from activities and stick to the 1350 that it allots me daily. If you upgrade to MFP premium you can tell it not to add those calories in, but I am too cheap for that.
Beth, mom to older DD (8/01) and younger DD (10/06) and always missing Leah (4/22 - 5/1/05)