Like just about anything else, the social networking site has pluses and minuses

Modern-day technological inventions can be used for both good and bad purposes. It all depends on how one perceives its basic functions — meaning the leaning should be towards benefits.
While questioning the legitimacy of a social networking site like Facebook one would have to apply the same guidelines. Logic should prevail over generalisation. This is highlighted in the current context of a recent fatwa being issued by an ex-official of Al Azhar against the immoral capacities of this networking site.
Essentially, Facebook's purpose is to serve as a platform for communication. And though it can contribute towards moral failings one should not lose sight that it has a primary purpose.
There is potential for misuse in most things. This exists when we use the telephone, write a letter, or send a text. But the primary purpose can never be cancelled out. Therefore, it all boils down to the intentions of the end user.