Ever since I was 15 (I'm 19 tomorrow ;D), I've never known how to make IRL friends, and I got very sucked into finding people to "make freinds" with over the internet.
After a while, I started travelling around to meet these people and I gained a lot of confidence with IRL encounters because I sort of knew the person I was meeting, but at the same time, it was the first time I'd seen them.
For me, it was a brilliant medium between the comfort zone of conversing with someone you knew for ages and trying to build a social relationship with someone totally new.
I digress slightly, but I have experienced that people do disappear, they become uninterested, or their life changes, or they just don't go online much...
The most common thing that causes this is the development of a human being. Someone once said, "It is a beautiful thing that if you, changing, are still able to love a changed person" and it is absolutely true. There are people that we click and connect with but somewhere down the line, over time, you will change, and just may not be compatible anymore.
People notice this, but they don't know what it is, but people do start to drift away or even disappear all together. It's a sad way to lose someone you had even the tiniest of relationships with, but possibly why it's better to try and move onto making friends with IRL people.