While facebook has access to a vast amount of data, there are also a few disadvantages of doing research on their dataset:

1. You only have data of people capable of buying a computer and having internet access. And then having the time and willingness to use facebook. This places a bais in the information and obtained research.

2. The profile of users don't reflect who they truly are. It represent what they want to be, or at least how they want to be percieved by others. Admittingly this problem is also present when doing surveys.

It certainly provides a lot more information and would aid social scientist, but it's not perfect. Then again they passively collect a huge pile of information too (e.g. using their like buttons all over the internet).