I've been thinking about this a lot and I have come to the conclusion that there are seasons in one's life. Sometimes we will have friends at one time and another season those friends will be gone. This doesn't mean that they were never really your friends. It just means that they and you went separate ways. So even though I first agreed I'm thinking that it isn't true anymore.
I think if they left in a way that hurt you that's a different story, but if you all just stopped talking because life got in the way, well that just means you all drifted apart. It's normal