SMO forms the bigger picture. Social bookmarking is just one part of SMO.
Social media is all about exposing your website on social media channels (social networking websites, blogs, video distribution channels etc).
On the other hand, Social bookmarking is a social media site for Internet users to store, organize, search, and manage bookmarks of web pages on the Internet.
SMO includes all social media sites including social bookmarking.
I agree with you.