You meet new people. You add them in facebook. You chat with them, tag them in pictures, comment on their status updates and share information.
Some of us even have our twitter account in our business card. So people may follow you and you may follow anyone whom you think is interesting and/or is informative.
An article published by Reuters says
The social networking phenomenon has nowhere to go but up as computer use becomes more mobile, according to leading figures in the development of the popular sites Facebook and Twitter.
To read more on the Reuters article, click here