OK, so WordPress.com is still by far the biggest single platform but there is fierce competition and they are making gains.
I remember back in the day when WordPress first came out – I disliked it from a code bloat and code purist point of view (I still hand-coded websites even when there were editors available) but I now have a love-hate relationship with WordPress.
The convenience and rich plug-in ecosystem is attractive, as well as giving clients the ability to easily make changes to their sites.
The Wix platform is one that has impressed me more recently, especially with all of the SEO improvements they have made.
Remember, at the end of the day the CMS nd web platform just delivers HTML (or javascript) to a web browser. Google understands that is is looking at a rendered page and treats it that way.
Read more on WordPress market share here