It seems to me that it’s less of a free software issue and more of a patents issue.
In effect, by using React, you agree to let Facebook and other React contributors infringe on any patents you may hold, whether or not they are related to React in any way. This only comes into effect if it is confirmed that React uses any patents held by Facebook or other React contributors though. As I understand it, this has not been confirmed so far.
I personally feel that this is not an ethical way to deal with patents. The Apache license, for instance, grants similar rights to what React grants without such strings attached.
Considering all the patent-related discussions and activism we engage in, I think it is necessary for FSMK to choose its stance on this issue before endorsing React in any way.
Edit: PS. IANAL. Pinch of salt recommended.