I just noticed google trends, where you can check the popularity of searches in the last few years. You can also compare searches. After checking out plone by itself, I did a zope plus plone comparison
I started with plain zope years ago, but I would use zope+plone+archetypes+archgenxml when starting now.
The plone.org documentation area plus plone's livesearch is so bloody handy that I rarely use google to search for plone-related information anymore. But, assuming I'm not completely alone in that, that could only account for a leveling-out in the last half year or so.
Google trends is not perfect, especially when searching for terms that have multiple meanings. Zope and plone are pretty safe. If you compare plone to CPS, CPS has twice the search volume. But when you see a news event next to the graph that says Some CPS workers have crime records you start to think that CPS is an acronym for many things :-)
My name is Reinout van Rees and I work a lot with Python (programming language) and Django (website framework). I live in The Netherlands and I'm happily married to Annie van Rees-Kooiman.
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plone.org traffic is 200 Gb, joomla.org is - 1 Tb
this is @#$!
I like Plone, we should make it hot!
Funnily enough searching for zope plone was the first thing I did too. ;)
What I think is more instructive is to have a look at zope plone versus some of the php based content management systems out there.
http://www.google.com/trends?q=zope+%7C+plone%2C+drupal%2C+joomla%2C+typo3%2C+wordpress
At the url above you can see searches for Zope or plone against drupal, joomla, typo3 and wordpress.
Joomla is the clear favourite and is also growing the fastest. Maybe this is because of the very public discussion about it forking from mambo.