That is a very interesting graph. The "Thanks Button" appears to have been introduced on 8/19/2010.
Here is the introductory thread by Toolman. I was never in favor of that button from its inception. As I stated then, it reminds me a lot of the "Like Button" on Facebook and IMHO, that button does nothing but make people lazy. Instead of actually writing a response, people can just lurk and click a button. I'm not saying it is the sole reason for the post decline, but it certainly hasn't helped.
Looking at the graph, the sharp decline starts in the fourth quarter of 2009 and has continued steadily ever since.
While I do feel that the thanks button has contributed to the decline in post count, I will also restate what I said in a discussion about this a year ago. Most of the dedicated members of the site are now (or have been) on the staff. There is absolutely nothing wrong with that, except when you find yourself with a limited amount of time to devote to the site and that time is spent moderating threads, BOD work, etc. That forces folks into doing work, rather than simply being a part of the community. In turn, the posts that those folks do make are fewer and less involved.
What is the answer? Well, to be honest, I don't know that there is an answer. Maybe a new flock of dedicated members is in the making.... As for getting the older members back? I think they'll come back if they want to, but you can't force them to do it. Taking the Thanks button away might help....
As far as an in-person meeting on the 9'th or 16'th, I will not be available. I may be available for a conference call during the week though.
Here are two more threads from last year where this was a topic of discussion:
http://www.ncwoodworker.net/forums/showthread.php?t=35549&highlight=button
http://www.ncwoodworker.net/forums/showthread.php?t=35740&highlight=decline