A thread is the chronological succession of a question and the comments that people post after it. When someone on the forum wants to ask a new question, they open a new thread and then people post comments on it. It makes a thread (like a piece of string) of comments.
I wonder if it doesn't have a relationship to the figurative definition 2c ("lost the thread of the argument"). It would be interesting to know how the terms "post" (as opposed to "reply") and "thread" achieved consensus and what that process was like.
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.