I was recently in a situation where I had to move and leave behind ALL of my high school friends and buddies that I knew a few years back. My parents couldn't keep up rent and had to move to another smaller city across state. Facebook helped, in that regard.
It breeds narcissism. It makes ordinary people think they are special because they have 1000 virtual friends who would never pull their car out of the ditch in the middle of the night if you asked. These are not true friends, rather they are a symbol of false popularity.
Also, social networking is really just data mining and people willing give their information away for free.
And they do not bring people closer together, before social networking you would go see someone if you wanted to 'keep in touch'. It was more personal, more intimate. You loose a lot of body language, mannerisms, hand gestures, tone of voice etc.
That the presidents of each of those networks are all puppets of the satanic Illuminati CIA and they attend Bilderberg meetings so they can be told how to spy on every individual who's soul is trapped in the occult virtual fantasy world.
You could turn it around on your teacher and argue that social networking is far from stupid: That the entepreneuers who invented these sites pretty much read the public's collective mind, played on the psyche of the masses, and turned a small idea into a monster. And look at all of us, feeding right into it! It's a hugely convenient way share things and stay in touch with everyone.
BUT that the HUGE downside to social networking is how dependant we've become on it. We're all slowly losing the ability to socialize normally, lots of people are reliant upon it to bolster their egos and build false confidence by posting hundreds of self-taken pictures making the "duck face", and it's replacing phone calls and visits. How long will it be until none of us can function in society anymore?
Turn it around, and do a rant about why your teacher is stupid for not realizing how facebook, twitter, etc, help society stay involved with one another
In today's society people don't have as much time to call each other up and stay connected, so social media jumped in to save the day