> Facebook is, indeed, the only giant I can think of that has not yet been disrupted

It's also still the youngest one of them, but yes, for a company in the social networking space they've showed a lot of staying power (they could've already fallen to what could've become out of independent WhatsApp & Instagram). Facebook (and Google) only exist as they do today because an acquisition by Yahoo didn't happen, and I think they've internalized that.