I came into the picture in August 2008. The DZ was in Full Tilt Boogie at that point, but I think it only started its DB Cooper thread in 2007. Jo Weber says she started it with Ckret/Larry Carr. Fact? Who knows. But Larry was posting regularly by the time I showed up and was joined by Sluggo, Snowmman, Georger, and 377. A few other posters were sharing their comments on a regular basis, but they are long-gone by now - Orange 1, Amazon, Hominid, Farflung, SafecrackingPLF, etc.
Sluggo's site was also up and running by 2008. I don't know when it started, but Sluggo said that he had been collecting facts, researching, and interviewing people for a very long time.
Galen might know more about the Ancient History of DBC online chatting. He has been following the case for decades.
For the record, the Mountain News-WA started in February 2011, and I think the DZ shut down during that year or 2012. I started posting DBC pieces on the MN in earnest by 2013, and my book first came out in 2015.