Hey boombox-
For me, the majority of my 'drinking-buddies' were just that. Just some buddies to drink with. Once I removed alcohol from my life, my drinking buddies and I had really nothing in common. I mean even meeting up while I only had non-booze drinks was no fun, as I learned the only thing we really had in common was drinking. So, as Purpleknight said, they seem to just go by the wayside.
Also, I really don't think you need to justify yourself to your buddies on why you no longer want to drink. If they give you some grief about it, I'd bet it's only because they just want a pal to go drink with.
When I stopped I use to invite some of my drinking buddies to play tennis, go for a run, bike or whatever. When I stopped the booze, they were really not too interested in doing the non drinking stuff. After a while, I've come to learn that most of my friends were in fact just people to drink with, and not much more.