Archbishop Vincent Nichols says that the "transient relationships" such sites are making teenagers to develop leave them unable to cope when their social networks collapse.
According to him, the Internet and mobile phones were "dehumanising" community life.
His views came after a 15-year-old schoolgirl killed herself by taking a fatal overdose of painkillers last week, after being bullied on Bebo, another networking site.
The Archbishop of Westminster also expressed his concern about the loss of loyalty, and the rise of individualism in British society, which he said threatened to undermine communities.
He said that the decline in face-to-face meetings and conversations over the phone was weakening relationships
"I think there's a worry that an excessive use or an almost exclusive use of text and emails means that as a society we're losing some of the ability to build interpersonal communication that's necessary for living together and building a community," the Telegraph quoted him as saying.
"We're losing social skills, the human interaction skills, how to read a person's mood, to read their body language, how to be patient until the moment is right to make or press a point.
"Too much exclusive use of electronic information dehumanises what is a very, very important part of community life and living together," he added.
Nichols further said that social network sites were leaving children with impoverished friendships.
"Facebook and MySpace might contribute towards communities, but I'm wary about it. It's not rounded communication so it won't build a rounded community," he said.
"If we mean by community a genuine growing together and a mutual sharing in an interest that is of some significance then it needs more than Facebook," he added.
Stressing that the sites were contributing to a trend for teenagers to put too much importance on the number of friends they have, he warned that it could eventually lead to suicide.
"Among young people often a key factor in them committing suicide is the trauma of transient relationships. They throw themselves into a friendship or network of friendships, then it collapses and they're desolate," he said.
He continued: "It's an all or nothing syndrome that you have to have in an attempt to shore up an identity; a collection of friends about whom you can talk and even boast. But friendship is not a commodity, friendship is something that is hard work and enduring when it's right."