As you say, wouldn't take a genius to come up with the idea.
Then there's another point how many people know FB is built on Linux? Bet you wouldn't have to take your mittens off to count how many it it's 8 million users do.

Not to mention the fact that neither LInux nor Open Source are entities, so gratitude is a null concept.

I'm a user, purveyor and proponent of both and I feel no gratitude whatsoever to FB or Ubuntu, that's why.

Can't think of a single reason why I should either. They espouse not one value of either, except free as in beer.