SEO (Search Engine Optimization) is a process of promotion a website (brand/product/service or Business), while SMO is not a direct promotional strategy, its focused on building reputation, creating a new market, brand building and more.
In theory SMO is not a direct means to market, it works like a blog (I mean a real blog) where the concept is building audience and improving a network and later use it to create leads (market).
In comparison SEO is simply promotion, more or less like a free ad (although a damn hard and time taking work), SEO however cannot create a new market as is the case with SMO...