Honest question, then what's the point of innovating, if Google, Facebook, or Twitter can simply copy your idea with an unlimited reach?
Google, Facebook, and Twitter are far from omnipotent. Want proof? Just look at Google's failed attempts to get into TV advertising or social networking. Or Facebook's attempts with Messenger. Or Twitter's... everything, really.
Also, consider, it's unlikely any of them would copy an idea until you've already made it a success, at which point network effects can work in your favour (again, Google's repeatedly aborted attempts in social networking are illustrative, here).
To summarize: they might try to copy you, but there's no reason to assume they'll succeed.
> "The best thing of all worlds is to build a product that a lot of people really love. In practice, you can't usually do that, because if there's an opportunity like that, Google or Facebook will do it. So there's like a limit to the area under the curve, of what you can build. So you can build something that a large number of users like a little bit, or a small number of users love a lot."
MSFT/GOOG etc... could always decide to "un-suck" and kill some new hot thing, but it's a deus ex machina and not something you can predict, so it shouldn't really even be a consideration.
I get where you're coming from but deep pockets and an army of engineers doesn't guarantee success. Google+ never got out of the "it sucks" stage into mainstream success. The legacy of Steve Jobs is also littered with fanciful ideas that never got traction. I guess my point is there is a point to innovating and attempting to try, even as a small player, because there's really no guarantees.
I can't recall the exact story right now, but I recently heard that a well-to-do / highly funded flight project (Langley?) failed, and a short time later, the shoe-string operation by the Wright Brothers was successful.
"MSFT/GOOG could just do it and kill your [service]" is a very common response to anyone who is launching anything new or groundbreaking and it's also a misguided response.
So worrying that BIGCO will do that is basically a worthless concern and saying that because something is obvious in retrospect that it was inevitable is not particularly useful.
That said, they still can do it and on the rare occasion when they do, instead of doing an acquisition it usually does kill the newcomer. So to the original point, it seems like it's actually becoming more common than less for them to spin something up rather than acquire.