I have no data. I only have the comments from the departing members of this guild and a Horde guild I was in at the time. In WoD both went from having around 40 people online on weekend days to… zero. The first few months of WoD were fine. People were leveling, doing their thing. Then about six months in, you could see/feel/hear the vibe change. Garrisons were up and running, and people were like - “this is it?”
The exodus started about six months in, and after ~2 months, most were gone.
I had no idea how much ore and herbs and other mats were flowing through this guild until WoD happened. These weren’t bots, they were casual players that were on a great deal of the time. When they were on, if they weren’t grouping to do old content they were sitting in Teamspeak. Flying around farming mats, working to level professions, or running old content.
If you weren’t good enough to be a “raider” you could be good at everything else the game had to offer. Sure your garrison was a great source of material for you. But prior to WoD there was a whole lot of stuff being traded back and forth between guild members. After the Garrisons came online that all died off.
See also this post with my further comment on WoD:
Edit - Spelling