Lost phone for years but number still active today?

[Tats]

I lost my phone 5 years ago, that was 2009 then, when i left it somewhere. I tried to call it hoping that someone with a good heart would give it back. But unfortunately, NO. My phone was turned off. ...So just this last January 2014 my friend messaged me on facebook. He said he tried to call me on my number registered to his phone (which is the one i lost) and someone picked it up saying she doesn't know me and my friend dialled the wrong number. I was shocked knowing that id lost it already and prolly the one who got my phone already threw my sim card.
Just now, as I was updating my account on a website, I saw that I registered there 2009 and i had placed my old digits in my profile. So I tried to dial it. It rang.

How did this happen? Somebody told me that there were people who recycles sim cards like it was a brand new number. Or, does TELCOs recycle numbers themselves??