Indexed pages going down, haven't made changes since April

Over the last few months my indexed pages in Google has been at a solid just under 13,000. This is out of 28,800 total.

Today I log in to Webmaster tools and notice that it's now reporting that I have 9,308 pages indexed. I haven't made any changes to VBSEO or my sitemap config since early April 2009. In April I trimmed down my sitemap to only include Showthread and forumdisplay pages.

Now I did notice that member profile pages are still showing in the SERPs.

Could the big drop in indexed pages be Google finally catching up, months later, to the changes I made to my sitemap back in April?

Any other explanation?

Chris,

I tell ya, you have a nice site bro. Hopefully my forum will be like that one day.

Thanks for the compliment!

Hi Chris, dropping the showpost pages is mostly like the cause of the drop and that's a good thing.

How is your overall traffic changing?