Just to clarify, the question is about announcing an offer (not a commitment) on social media, right? Just confirming bec the title says "announcing commitment".
Like some mentioned here, I've always wondered why other sports announces offers on social media. I personally much prefer baseball's culture where it's not done this way (although I won't be surprised if this changes later on).
What I've been told is that college baseball is a small community. The moment the first offers comes, other coaches will know somehow and that could open the door for other coaches to offer (either they have been interested but waiting on the sidelines, or learning of the offer brought the kid to their attention). I'm sure the travel/HS coach plays a part in making sure that their college contacts learn of this offer through back channels.