Facebook now makes around $1B per year in revenues. So does Zynga which develops social games on platforms such as Facebook. I would say Facebook has become a solid platform worth devloping for without too much of a platform risk.
It is extremely risky for companies to build on platforms that are still searching for business model (read twitter). I am not saying that companies can not be successful building twitter based products and services. At this moment it comes with a lot of risk. Facebook platform has significantly reduced its platform risk.