Against
You can make a friend anywhere. Doesn't necessarily mean the relationship has to stay soley online.
Many people are 'friends' only because of proximity. Most of your colleagues, classmates and neighbors are not your friends yet people call them friends. Once you or they move, often you are no longer friends. The internet doesn't have proximity issues as anyone anywhere can be your friend and not just because of convienience.
You can filter your friends better by common interests online. You can also maintain a friendship easier. You can make friends online before you partake in a certain activity in a certain location, you can also choose who you associate with more carefully.