I'm a member on AROC forum and post regularly, the classic sections are more active than the modern areas it really is a brilliant source of knowledgeable, often professional people.
AlfaBB is also excellent for technical stuff on the older models, but is largely USA based.
Pistonheads is OK but there's not all that much Alfa related stuff going on.
AO is still the best forum for general chatty stuff and tech details on recent Alfa's.