Search Engine Optimization (SEO) means preparing your website for Search Engines. With Search Engine Optimization,a company website can be found in the "organic" or non-paid rankings of search engine results for various keywords. Not all keywords will result in your website being on the 1st or 2nd page of leading search engines like Google, Yahoo, and Bing.
I say that a website without Search Engine Optimization is like a car without gasoline. It's dead in the middle of the Internet superhighway, going nowhere, bringing no one to it, and doing nothing of much. When a website is optimized, it becomes the gasoline that allows the website to run on the world wide web.
Social Media Marketing on the other hand is a strategy used to share content, invite interactions, and communicate key messages about your company to get visibility. With Social Media Marketing you get a "buzz". Popular websites like Facebook and Twitter allow quick sharing of content which drives traffic and brand visibility.
To get succesful online, you cannot choose one or the other. When done properly, Search Engine Optimization and Social Media Marketing can help you maximize your online investment and increase your visibility.
Businesses doing only one or not doing anything with their websites have no chance of being found.