Things like the prom stuff are his, I guess. Other posts like '36 years ago we recorded Good Vibrations" are more likely to be written by staff.
I agree, Mr. Bear. On quick glancing it doesn't seem like everything's written by Mike, roughly it's 60 to 40 (1st being Mike). But I have no doubt he can manage to write basic SMS/WhatsApp message, if we talk about Mike's tech stuff abilities in general, not just FB.