There are several reasons why Word Press is so popular. One of the mean reasons is that allows you to roll out your own blog and can also be used to run anything from a forum to full scale eCommerce store. Word press biggest competition is tumbler but it’s still full way back.
Yes, and I think the number is growing in terms of having sites made entirely from WordPress. Popular sites like mashable or CNN and many more used WordPress. SEO benefits is a charm when it comes to WordPress as Google actually crawls from descriptions then only tags and other keywords and compile the related material in WordPress.