I've had several dear online friends for years now, and these are my personal conclusions.
Pros: They are friends. The fact that they live far away from you doesn't change your feelings, nor theirs, if you grew close. I have a close online friend who I started talking to back in 2011. I was very depressed back then, and I didn't have too many real life friends. Then I met Derek, who lives all the way in Northern Ireland, and as it turned out, he was also depressed. But he had that never dying sense of humor which I needed, as I soon realised. The way he'd make things sound funny, and his wit would make my day much better and lighter, despite daily anxiety and stress. And I had the kind of interests and topics I liked talking about that made his days better, despite a family tragedy and health problems. I didn't even have Skype, nor a good PC. Just a phone and messages, and we messaged each other every day. Message by message, and he got to know me better than my family, and one or two friends I had at the time. And I got to know him. Today, when I think of 2011, that online friend is one of the brightest memories about that year, not just because not many pretty things happened at the time, but because meeting him was a great thing, and would remain so even I had twenty great real life friends back then. He'd still remain a great memory, because our friendship was great. And it still is, in 2016. Even if we get to talk once a month, the closeness will always be there. And so will the gratefulness.
My other online friends of who some are quite close to my heart live all over the globe. One lives in Africa, and I am in Europe. Another one lives in Hollad, another one in Germany. Once I had one from India. That's another great thing. I got to get to know a person that I in normal conditions would never meet at all. Thanks to online friendships, I met an African and his way of life, his different ways of living, a brand new world of a person from a completely different continent. Getting to know people who live so differently than you and becoming friends with them is a rich blessing and a rich experience.
Cons: You can become such great friends that the distance becomes a painful thing to remember. You can't be close to them and offer them a hot cup of tea when they need it. Also, the danger of the Internet urges you to be much more careful when trusting someone, so that's something to care about. You only get to see the side of them which they choose to show. But it can happen with real life friends, too. That's one more common trait of both friendships. Online friends are just people whom you haven't hugged yet. :)