I think one of the reasons activity has died down is because the GPL now "competes" directly with another popular GP web forum. The two have essentially the same topics, so it's easier to read the site with the higher traffic.

The email GPL was successful despite GP web forums because it had a niche. Email was a different medium than web forums; you could follow the GPL passively and it seemed more personal. Now the web GPL is redundant with other forums, no flame intended.