The most disruptive thing about gmail was that it was obscenely expansive webmail space compared to your typical webmail provider. But webmail was already pretty damn common when gmail came out, and email had been the killer app of the internet for a decade already.

Facebook Mobile is... I'd have to think hard about what it actually is, so I'm not sure that qualifies as disruptive.

Now the iPhone was disruptive. But although you'd fairly classify Apple today as an "established mega-corporation", it's a lot harder to do so when the iPhone came out. To the extent that Apple can use its market share to wield a powerful bludgeon against those who dare cross it--which is what I associate the phrase with--that market share doesn't exist until the phenomenal success of the iPhone. It certainly couldn't do that on the basis of its OS or computers (indeed, you can argue that it still can't do that today). The iPod, or more accurately iTunes, may have given it that power against the music industry, but that's the closest to any sort of tyrannical power it might have held at the time.