>is nothing more than a "fad".
Don't kid yourself. Previous internet enterprises may have been passing fads, but that doesn't mean that FB will fade out.
A combination of timing and scale means that FB will be in position to beat off any challenge (copy key functions or simply acquire the company). It may also be big enough to buy politicians, to avert any legislation it doesn't like.
A lot of people thought the telephone was a fad. TV, too. And the WWW, I seem to remember.