Facebook Inc. went the public funding way by offering its shares to public through Initial Public Offering in 2012. We will go the simple way to get current Facebook Valuation based on market capitalization.
Since Facebook is now a public listed company so we can easily get the current market capitalization figure. The formula to get the figure is by Multiplying Total Outstanding Shares (latest) with the current Market Price.
To simplify the terms:
Latest Shares Outstanding = 2.55 Billions
Current Market Price per share = $ 69 (As on 18 Mar 2014)
So Total Market Cap = 2.55 * 69
= $ 175.95 Billions
For professional valuations we consider Enterprise Value as being a preferred measure to get correct valuation which requires close scrutiny of filings & latest earnings report.