After some time spent on investigating this very interesting query, I was unable to locate the first ever thread posted in the forum. I initially looked up the scoreboards of the Webmasters, but their first posted threads were much later than when ISC was launched in 2007. That year, I was not a member so could not find anything via my own scoreboard either.
Then I looked up the Net on how to locate earlier versions of a website. Accordingly, I did come up with a snapshot of what ISC was like in 2007. It seems that in the initial months, there was no forum at all! There was at the start, universities, courses, question papers and later a resource section. The initial resource section had write-ups which were not on the same lines as the present article section, but more like the forum threads and, in fact, the word 'forum' appeared against a member's contributions' list but the URL showed the word 'resource'.
Perhaps Tony Sir would be able to solve this intriguing mystery of the first forum thread!
When people come at you with their worst, you should come at them with your best (advice given to Selena Gomez by her mother, quoted in Time magazine.)