FWIW, I've seen a few stories of people - nobodies - sending a letter as a thank for the knowledge provided, the inspiration or what not and getting a genuine letter back. Obviously not a whole spiel, but a sentence or two indicating intent. Despite their massive notoriety, I wouldn't expect he's getting swamped by letters (tho I could be wrong) so I'd find it quite plausible and in-character he'll take the time to read a handful a week (a lifetime of 10-Ks would make anyone a proficient reader) and write back (I guess with some templates to go off from). I'd be surprised if sending a letter to Meta reaches Mark - tho maybe, I guess he receives fewer thankful ones - but that lean HQ culture of Berkshire seems rather unique with a measly single and modestly sized floor to manage one of largest balance sheets out there.