He probably realizes that using his song for endorsement of Instagram would have deceived a lot of fans.
Maybe the prevailing demographics of HN is getting too young to get it, but FB asking Roger Waters about nothing less than Another Brick in the Wall, Pt. 2 to sell more of their business... that's either being cynically obtuse or being simply too dumb. In any case it's a waste of time for everyone.
I agree that it is tone deaf to ask Roger to support Facebook, but if Zuckerberg somehow got the rights to the music, some fans could be fooled.
For example, my first Pink Floyd album was Atom Heart Mother, and at that point I didn't realize that Pink Floyd/Roger Waters went deeper than the music.