Interesting topic - friends versus colleagues.....one that I am grappling with right now. We've moved back to the USA - are living with family until our house is built far away from where we are now. So we are in a long transition - I've lived in the city we are building our house before and have notified some of my friends from before....and hope to be friends again, but it might not be so. In the meantime, some of the people we socialized with in Germany we've drifted away from (military, so also a few moves by others and us causing the drift as well).
I have several friends I've met through my different work environments that I consider true friends.....and I do keep in touch with some of my high school friends, but frankly there is little I have in common with them except the memories. Heck, I probably have more in common with a lot of you here and would consider you closer friends, in that respect.
I like having friends and socializing, but not too much. Work is work, but sometimes you meet someone that can cross the threshold to friend - I hope that I can continue to be friends with some former colleagues...some of the others - drift away.