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.
The most disruptive thing about Gmail had @&$@ all to do with user features. It just needed to be "good enough".
The key feature of Gmail was scale, which allowed it to offer much better spam detection, which allowed it to capture more market share, etc.
Facebook mobile is the idea that you have something (Facebook) that was popular somewhere (PCs), that for strategic reasons (TAM / global penetration, location-targeted advertising) pivoted to a completely different medium, and invested non-trivial company resources in making that happen.
The iPod/iTunes were pretty damn big. And specifically, big in demographics that were ideal iPhone customers. Apple in 2007 was very different than Apple in 1997.
Gmail was an already popular concept whose differentiator was a company was willing to set money on fire to have two or three orders of magnitude more storage than their competitor. AFAIK, that's been their only differentiator - free storage space.
iPhone was a breakout product from a decidedly no-longer mega corp who practically died in the 1990s. I guess they had already made a resurgence with the iPod, but I think iPod leading to iPhone is similar enough that it's the same jump.
A mobile app from a company is a breakthrough product in your eyes?
There was a time when Facebook existed primarily on the PC web. Everyone thought Zuckerberg was dumb for pivoting substantial company resources into mobile (initially HTML5, then pivot to native app).
Edit: To be clear, IMO most disruptive technologies are not driven by established mega-corporations.