Regular investors were shut out of FB's initial IPO. So you didn't invest in shit OP.
And their debut was a failure, they needed to get underwriter's to prop up their stock, it barely broke even. Anyone with FB stock will right now will be disappointed in a couple months.
The main reason why you shouldn't invest with FB is, that unlike a company like McDonald's which will definitely be here in 5, 10, 20 years and has a very low chance of a competitor wiping them out, FB can be gone as quickly as they become uncool. FB is desperately trying to expand, you might see websites replacing their comment system with FB comments, and to login to certain websites all you need is your FB details. But in the end they will be like MySpace. Things that are cool favor the new. That's why G+ failed, no one wants a profile page with a search engine company.