I've hit an interesting article in Wikipedia called Comparison of BSD operating systems
The article explains basic difference between different BSD (Berkley Software Distributions) and what is the primary accent of each of the BSD (free software OS) distributions. It also reveals basic details about the history and how each of the BSD's came to existence. I recommend to anyone interested in free software as it is just a great reading for everybody interested in FOSS.
The most interesting part of the wiki thread is a bar chart, provided by BSD Certification Group research conducted in September 2005.
The above diagram is showing the proportion of users of each BSD variant from the BSD usage survey prior conducted
The research is already 6 years old, and unfortunately as of time of writting seems to be the only publicly available. Though being outdated, I believe generally the bar charts distributions along different BSD variants would be mostly true. The only big difference will be probably in PC-BSD which is not even on the diagram should have outbeaten DragonflyBSD's use. Since there is no public data available for 2012 and the years 2005 – 2012 for the use percantage of each of the BSD distributions, I've thought about a pseudo way to get some general statistics on each of the BSD distributions popularity. The methodology to gather the required statistics comes to simply, type in Google each of the BSD variant "code names" (e.g. freebsd, netbsd, openbsd etc.) and look at the number of results returned. It seems logical the more results distribution keyword searched returns, the bigger the probability of more users to be involved in developing or using the respective BSD variant.
Below you see the results, I've gathered in my quick "google research":
As you can see in the above data FreeBSD is still probably leading the BSD use, the public interest to OpenBSD – BSD focused on security has significantly grow since the last 6 years. Next it is seen the PC-BSD users base has probably tremendously increased and according to the Google results returned it is probably on a 3rd place by users interest (use?) followed by NetBSD with only 1.47% of all the BSD users. Lastly with only 0.99%, orders Dragonfly BSD which no longer is so popular as a Desktop BSD based OS as it used to be back in 2005.
Again the presented diagram results are based on only on the factor of Google BSD variant popularity and hence shouldn't be consired too trustworthy, still I'm sure it gives a general idea on how used is each of the BSD variants as of Jan 2012.