Facebook terms of use say that you grant them rights to use any photograph you post in their advertising or, really, however they want. If you license the image you don't have the right to grant usage rights to a third party. So, theoretically, someone can't license the image from a microstock agency and use it on Facebook. At least that's how I understand it.
When I post my own work on Facebook I watermark it thoughoughly and make it very low resolution because I can't keep Facebook or other Facebook users from using it or distributing it.