When a new area of technology, etc, requires a lot of vocabulary, people fall back on what they know.
Post and
thread seem logical, but
spam comes from a (not particularly funny IMO)
Monty Python sketch. Dialect words can resurface at times like these- you can find them in both Australian English and drug language, both areas where new experiences required terms for new circumstances. Consensus is built through usage- prepositions, for instance, are often unsure at first, but eventually one wins as consistency is a good thing. When things are changing rapidly and you need new terms, to look around for what you can remember and if others agree, they take hold. Forums offer linked messages in a new way- thread seemed a good term as there's a connection between the messages. It took off and won.