Wikipedia, it is available in the format of a free codec. It is also available under a free license, a license that insists that whatever you do to the video, it has to remain available under the same conditions.
I am not a lawyer but my understanding is that the consequence is that a conversion to a codec that does not allow for commercial use like the MPEG does, is not allowed under the CC-by-sa license.
Facebook intends to host Wikipedia content and consequently it will have to deal with this. It will be interesting to learn how they will twist and turn in order to deal with this issue.