I've only had three best friends in my life.
The first was a boy who lived across the street from me. We went to Kindergarden together and basically did just about everything together up until the point he was going to move to a place I have forgotten since. It was about two years after we became friends. I found out that he was talking negatively before he moved, but didn't remain in contact with him until the point where I had to say goodbye. My first best friend-best friend relationship: broken.
My second best friend basically took his place rather quickly when I entered second grade. He moved from a different school. We remained best friends for a while up until middle school where he started to irritate me. However, our friendship continued towards high school. We were juniors (well, are juniors) when our friendship ended. He doesn't know, but I found out he smoked and didn't want to be around such a negative influence. You see, I have asthma, and when I'm around smoke it gets really bad. I was at a homecoming meeting with our class (graduating class), and he came. He didn't go to the same school as me since he decided to drop out as well and he came with cigarettes and smoked RIGHT next to me while saying how much he loved to smoke. I clearly remembered just a few years back when he said he'd never do that. We both promised each other when we found out another friend smoked. He broke that and our friendship and is now someone I don't want mentioned in any conversation between me and someone else. My second best friend-best friend relationship: deceived.
My final best friend relationship was with someone who moved to my neighborhood from Florida and has sense moved back. This one was actually my strongest friendship I ever had with someone else. I was introduced to him by someone I didn't really care for since he was a known trouble maker around my neighborhood. He was riding bikes with him and we met up. I said hi, and then quickly went back home across the street. The next day came, and I saw him riding alone around the apartment complex across the street. I continued to meet up with him on strange occasions leading to the assumption that it was probably meant to happen that I'd be friends with him. He was there for me everyday after that. I was his first friend there and his first friend to ever enter his house. His parents knew me, respected me, and treated me as their own since I basically lived in thier house for the majority of the two summer vacations to follow. He was there for me when I got my bike stolen, I was there for him when his dog ran away. Up until 8th grade, everything was going fine until I found out that his dad got a new management job ALL the way back down in Florida. This one was especially hard for me to cope with because we promised to keep in contact with eachother but it seems he cut relationship slowly with EVERYONE from my neighborhood. My final best friend-best friend relationship: nullified.
I do, however, have one friend that I had been friends with since pre-K. We met at a day care center and are still friends. However, this isn't a best-friend status. But, with that said, I do believe friendships can last as long as you keep the friendship close to you and don't let anything interfere with it. But that's about it. So, forever: no.
In terms of people online, no. I don't believe online relationships (unless acted upon into a real life relationship) can last that long. I do believe they can last for a number of years (ranging up to about ten), but forever? Can't say I think that can happen.