I think the additional $2 billion in value was created by showing they have a strong enough brand that a competitor can't throw together a clone quickly and take away their market share.
Snapchat's monetization potential is staggeringly limited. Snapchat adds ads, they annoy people, everyone moves to Poke or some other platform which does the same thing.
Facebook at least, have a way to use the data from Snapchat to add value to their other products - Snapchat, not so much.
