What’s the word for somebody you know (sometimes well) but who isn’t your friend?

Certainly acquaintance would be my own choice, but perhaps you found something remiss with it.

You might further qualify it by saying something like casual acquaintance, incidental acquaintance, nodding acquaintance, or long-time acquaintance.

Of those, I rather like nodding acquaintance, since it implies that this is someone whom you would nod to in passing, but not pause and chatter at.

The word is acquaintance.

2 a person one knows slightly, but who is not a close friend:
a wide circle of friends and acquaintances

[ODO]