I may be wrong, but it looks to me that community-upvoted advertisements of open-source projects are never displayed on Stackoverflow site. At least, I don't remember seeing any of these ads on the sidebar where this question says they should appear.
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And we're back... We'll keep an eye on them and may tweak the impression numbers a little, but they're running again.
The ad server has a whole bunch of priorities that you can look at as cascading buckets where impressions are first taken by the highest priority, with the remaining falling through to the priority below etc etc. What happened was that all available impressions where being used up by higher priorities, leaving none available for this campaign. As a fix I've moved it to a higher priority. To catch these things going forward we'll actually start treating ourselves like a client, and run recurring reports on our own ads to see what should be running (and whether it actually is...) rather than trusting they are...
I did a test. Simple test. I refreshed Stack Overflow homepage about 100 times.
Based on this, I would say there is a bug here preventing open source ads (there are four eligible now) plus the "third party" ads that I used to see once, from appearing. Surprised those who pay for those ads to show didn't notice that first. (unless of course there are no more paying advertisers, which I doubt)
Tried both logged in and as anonymous visitor.
Yes, they are displayed, see Open Source Advertising - Sidebar - 2H 2013
However, there is a score threshold of 6 votes before an ad posted there is considered for rotation at all, and there is currently only one ad eligible. It received 25 clicks at the time of posting, according to that page, so presumably someone sees these.
With only one OSS advertisement in rotation, perhaps you missed it?
Not exactly an answer, but some more observations. I participated in the previous half year program and once working on the SEO of my site I incidentally found an open source blog that demonstrated the banners from the Stack Overflow program.
And if there is one, then probably there are more such sites.
So, the banner clicks showed in the statistics, possibly are generated on an 3rd party sites, willing to advertise open source projects.