We at Black Duck noticed it too. So far, looks like the decline is largely because, lately Ohloh - a) hasn't kept up with discovering new projects. b) hasn't paid needed attention to keeping existing projects up to date.
We'll provide more details once transition is over (we just acquired Ohloh from Geeknet).
I suspect it has more to do with the mechanism by which commit volume is measured than the actual commits themselves.
It's possible people are just moving away from verbose languages like C and Java and thus just needing to write less code, but I'm not sure I have enough faith in the human race to believe it.
Another possible reason is that as systems become more mature they require less maintenance. I know the 2 or 3 projects that I am involved with are pretty mature right now and we are mostly just fixing bugs.
One theory: programmer utilization at companies has increased with the recession, so there is less time for open-source commits.
Another theory: a huge fraction of open-source in recent years is written by programmers paid by companies to work on it. The recession has caused these programmers to be pulled off and put on projects where their time can be billed directly.
Third theory, there is more use of distributed version control "offline", which would not be detected by Ohloh.
Well, in contrast to the second theory, there's the possibility that open source commits and closed source commits look pretty much the same. They're mostly done by people looking to get a paycheck, and without a paycheck the quantity drops. People aren't necessarily being pulled off open source to work on closed source, they're just being laid off (which seems supported by the unemployment stats.)
They're tracking any open source project that's registered on their website. If you have or know about an open source project that's not on Ohloh, you'll have to register it and specify its repositories (and branches).
Well, then a possible explanation is that people don't submit much projects to their website anymore and thus there are only "old" projects that are getting more and more mature and thus needing less commits.
I couldn't find any figure to back this though.
But I do remember that Ohloh used to be quite popular and I'm sure as hell that it's not anymore.
I wondered what exactly was being measured here too. I checked around the site quickly but found no answers. Bah. Might as well be made up numbers as far as I'm concerned.
What about the possibility of the emergence of distributed version control? Perhaps this is causing less actual commits to the actual project, but not necessarily less code being written.
No. And no. The initial link was in Values, not in Percentage. When you click Update, the graph shown is switched to Percentage. Here is your link in Value:
HTML, JS, CSS, PHP, and Python all have steady increases, and ruby is mostly stable. It could just be that focus is more on the web (HTML in particular seems to have taken a biggish jump mid-2009 - related to HTML5?)
http://www.ohloh.net/p/numpy