The value to FB/GOOG is the audience -- the users they're trying to reach. It's not as if they couldn't replicate the technology to drive that type of service. The defensibility of Snapchat against competitors is the demographics of the user base.
I don't use Snapchat, nor does my wife. Or any other set of parents I know. But my kids do, as do their kids. Right now, for any teen/tween in our community, it's Snapchat and Instagram.
And if I acquire Snapchat, that gets me.......what? A service with no revenue and a user base that's naturally fickle. The only way I can justify this acquisition for FB or GOOG is if there is some long-term value from the user base. And I don't think the value is there.
If I'm GOOG or FB, I think I sit on Snapchat for as long as possible. I'm betting that Snapchat's value will never be more than it is right now.