AGAINST.
1- You can make friends online and eventually know them in real life.
2- A friend is not measured by if they know you in person or online but personally, things about you other than how you look, how your voice sounds like or any other things only real life friends can know.
3-You can choose if you keep being friends with someone online by just ignoring them, real life friends always demand more courtesy.
4-You can choose WHO you are friends with online, real life friends always seem to appear and it's difficult getting rid of them.
5-Real life friends sometimes only care about if you buy them beers, help them with real life stuff, like lend them your phone to make calls, lend them money, or in few words, making them favors and if you don't do them favors, you're a bad person.
6-Online friendship is simplier, you and the other person just worry about enjoying the company of one another.