Indexed Pages Going Down

One of our clients has seen a gradual dip in number of indexed pages since the second half of October. So, here's a bit more about the client's site:
* Site has over 50,000 URLs. It's a database driven site (not a directory and not an e-commerce store). It contains information of member companies including profiles of their executives. It does not sell links (we know a lot of sites doing this suffered the last PR update)
* The site does not have duplicate content nor does it have any other domain name / alias
* The site has been around for over an year and a half and has never had any issues with Google in the past.

This is a syndrome we have detected with another client of ours who has an open source software products website...also a large site with forums that create user generated content every day.

I have also heard of one other site (not a client), who also has a large site and has suffered a dip in number of indexed pages and a resultant dip in organic traffic.

Does anyone know of any recent updates in the algorithm that are causing this? Has anyone else experienced a similar thing?

Thanks in advance for the insights and views.

Tushar

I am by no means experienced, and I'm sure one of the experts here will be able to give a better answer, but I'd like to share with you my experience on counting the number of indexed pages

The method I use to check my indexed pages in Google is site:myurl.com, and then on the top right a number comes up. But I've never found this to be a true number - if you keep going down the list of pages, the further on the results you are, the lower that number starts to get, until I would get to 450, where it ended. However, on the first site: search page, it said 2,090 (this is with omitted results included)

If you are using the same method to check the number of indexed pages, it's possible that the number you see on the first page also isn't a true number, and therefore can change from time to time.

Again, I know this is not a good answer to your question, but I thought I would share my experience on this topic, however limited it is.