At the time of writing, Facebook is reporting to have more than 750 million active users. If the Facebook population was counted as a country, it would be the third largest in the world behind China and India. It needs no more introduction for almost any organisation to see its potential.
Facebook was originally setup in 2004 for students, by a student. Since then, it has grown to be used by almost any demographic. The following facts are correct as of the time of writing, but the bottom line essentially is that there will be someone on Facebook that is interested in what your business or organisation has to offer.