If you patent a compression algorithm, and Facebook infringes, and you are also using React, and you sue Facebook over your compression algorithm patent then:

Your license to any patents which Faceboook may or may not have that cover elements of React will terminate. At that point, Facebook may choose to initiate a lawsuit against you over your infringement of whatever React patents (if any) they have.

So if you have (or are planning on obtaining) software patents, the React patent grant may not be very valuable. (Then again, Facebook may not have any patents actually covered by the grant, in which case it's not valuable to anyone.)