You can read blogs, documentation and stats about the competing frameworks to determine which technologies to use, or, you can see what others are actually using.
The following images compare ASP, ASP.net, PHP and Joomla. These three comparisons show that although there is a decrease in PHP (which is what Joomla and Drupal are written with) there is an overall upward trend with Joomla compared to others which look as though they are slowly decreasing. Although not shown, Drupal and WordPress also show an increase to their usage against asp.net. These results are as of July 26/09 from Google Trends. We have all learned to use and trust Google, so are these stats about as accurate as we can get?
Although it looks like PHP is on the decrease, the question would be why, when the big 3; Joomla, WordPress and Drupal are on the rise. Could that be an obsolescence in other PHP scripts that had once been popular?
ASP.NET vs JOOMLA
ASP vs PHP
PHP vs ASP.NET
All images are from Google trends