As Wikipedia has an Alexa rank of 6, and is the most popular web site I know of that gives an occasional survey of its server logs, I tend to look at it for information on web clients. Like their October 2012 report
( http://stats.wikimedia.org/wikimedia/squids/SquidReportOpera... )
For Linux client systems that use Wikipedia, Linux kernels with Android dominate. There's about 7 Android clients for every 1 Ubuntu client. There's about 45 Ubuntu clients for every Fedora and SuSE client. There's 2 Fedora users for every Debian user. And so on - less used Linux clients are listed on the report as well.
Netcraft used to have a decent survey of which web servers were running Linux, but I don't see any recent reports from them on that. With the rise of EC2 and PAAS/IAAS I would guess the Amazon Linux AMI would be one of the new VPSs/instances/"servers" that is around more nowadays.