How do analysts calculate the real $$$ value of Facebook as being in the billions?

[John W]

Once a company is public and it's shares are traded then it's value is the total number of shares issued multiplied by the share price provided that the market is efficient. As to determining value from cash flow, there are a lot of assumptions to be made as to how those cash flows are to be continued and increased but if you could know the future cash flows then a comparative value to other investment opportunities can be made with discounted cash flow.