I've never had this happen in real life, though I had an online friend who did it once. She and I were best friends, and after five years we finally started drifting a bit, but it was barely anything to stop being friends over. She just got more into the video game fandoms, and roleplaying on different sites, and did her own thing. I did my own thing, but we still leveled a lot, talked every day, etc.
Then one day she just never came on MSN, or anything ever again. She made a new MSN, deleted me on facebook, and all of the other sites she had me added on. I asked a few friends, and they said she'd done the same with them, but at least still had them on facebook. She just ignored them. We guessed that she got tired of us one day, and moved on. -w- This is why online friends are risky to me, and something somewhat similar happen to me IRL come to think of it. Friendships in general, I'm never sure how strong they are.