CentOS is a Red Hat based free operating system which enjoys widespread use among servers. It does not have the recognition of Ubuntu, Fedora etc. since it focuses entirely on servers not on desktops.
According to a report from W3 Techs, CentOS is now the most popular linux distro used in web servers. It has overtaken Debian which is now at second place. According to their statistics, CentOS is used 30% of the linux based web servers.
As you can see, CentOS has been gaining popularity quite rapidly. Ubuntu is also enjoying a little increase in server deployment. There is very little change for Debian, Gentoo and SUSE. However, Red Hat and Fedora are having a decline in popularity.
W3 Techs also published details of which distros lost out to CentOS and by how much. According to their statistics, CentOS is gaining primarily from Red Hat and Fedora. While 5.03% of the servers which are using CentOS now was running on Red Hat and 1.53% of the CentOS users were using Fedora.
Another interesting thing is that CentOS is not as popular in large websites as compared to smaller websites. While 9.7% of the top 100,000 websites uses CentOS, only 6% of the top 1000 use it.
We cannot say how accurate these statistics are, but yes CentOS has been gaining popularity in server deployment recently.