I thought he exposed the identity of an undercover CIA agent. That seems like a fact that was supposed to be secret, which he leaked (supposedly.)
You're right, he was convicted of obstructing the investigation about how Valerie Plame's identity was leaked... so it's not fair to say that he leaked it himself. He just protected Dick Cheney, who - semantics aside - was likely the actual leaker (in terms of responsibility for the leak and directing that the leak occur).
Libby didn't do time. His sentence was commuted by Pres. Bush before it started iirc.