1) More time
When people met each other somewhere. They travel to their meeting and they travel from their meeting. Also they have to pay fare. This things consume time or money. While they are traveling, people on computers discuss on Fb (abbreviation of Facebook). Moreover, they can talk to more people than just that right one. They can feel more comfortable at home more than people somewhere in the centre of city or worse in subway. They have everything what they need. Because f2f meetings are problematic due to travel, fare comfortable people prefer interaction on Internet.
2) New friends
Social networks let people meet more and more people. When you were acquainted with some nice girl through mutual friend. It's easy to find her on Fb and talk to her. Adding her on Facebook encourage to talk more and more to her via chat. Maybe there will be some final appointment but the chat will predominate. Hence people sometimes prefer acquaintance via Fb than normal way online interaction is growing.
3) Scheduling
One of the point which I prefer is easy scheduling through facebook. It is hard to schedule something at evening without Internet or Phone even harder with f2f interaction. Is easy to create facebook group and invite certain people to come. Other way they can contact them on chat. This way is much easier then to let particular people know by mutual friends or come to them by car or bus. Because of the scheduling
4) Fear of communication
As we all know we are not same people. Someone is afraid of spiders and some is shy when he speaks in public or to very important person[1] because of gestures, stammering and caring of oneself outfit and so on. It doesn't matter on Fb or in Internet at all. This connects to point 2. People choose facebook to dispose of shyness. And they leave the effort to break shyness on public. Thus people more and more tend to Social networks to avoid shy f2f interactions are declining.
5) convenient and more