I understand Facebook's strategy trying to stay on top of the mobile social game and remaining relevant to young users.
While I value Snapchat from Facebook's perspective, I think Snapchat itself is now in a tough spot. They're going to raise money at a valuation higher than $3b, and soon enough, the pressure will be on to start generating revenue. It's funny - people give social apps like Snapchat a hard time about not making any money, but if they tried to monetize right now, they'd look like idiots for thinking they're worth $5b or whatever, because the revenue would be peanuts in comparison. They're better off literally not making anything for the time being.
There's no way they'll ever get $3b out of that business without selling out. Maybe they think FB or someone else will be back with a better offer. I'm not sure that gamble was worth the risk.