Facebook and Twitter (initial question)

What is the purpose of facebook and twitter? People seem to use them for all sorts of things.  Are they networking tools or gossip columns. Is it a public notice board or man's desperate attempt at having a neighbor/community when he isn't satisfied with the people living to his right and to his left.

I signed on to Facebook a little less than a year ago and rarely signed in until lent when I decided I should make a better effort to stay in touch with old friends and acquaintances. I am told facebook is a good way to do this and the idea of logging in yet another website didn't appeal to me so what better way to mortify myself. Now, twitter is all the rage and I suppose I should go check that out too. 

I am interested in the moral aspect of these sites. What is proper etiquette? What is socially acceptable or expected?  Is there any code for behavior other then my own conscience? I am not willing to condemn such networking sites universally but surely there are attitudes and behaviors of the participants make them more or less constructive.  What are its virtues and what are its vices?

Any thoughts?