The goal of SEO is to get a web page high search engine ranking. The better a web page's search engine optimization, the higher a ranking it will achieve in search result listings. (Note that SEO is not the only factor that determines search engine page ranks.) This is especially critical because most people who use search engines only look at the first page or two of the search results, so for a page to get high traffic from a search engine, it has to be listed on those first two pages, and the higher the rank, the closer a page is to the number one listing, the better.
Yelp consists of a comprehensive online index of business profiles. Businesses are searchable by location, similar to Yellow Pages. The website is operational in seven different countries, including the United States and Canada. Business account holders are allowed to create, share, and edit business profiles. They may post information such as the business location, contact information, pictures, and service information. The website further allows individuals to write, post reviews about businesses, and rate them on a five-point scale. Messaging and talk features are further made available for general members of the website, serving to guide thoughts and opinions.
Facebook pages are far more detailed than Twitter accounts. They allow a product to provide videos, photos, longer descriptions, and testimonials where followers can comment on the product pages for others to see. Facebook can link back to the product's Twitter page, as well as send out event reminders. As of May 2015, 93% of businesses marketers use Facebook to promote their brand. A study from 2011 attributed 84% of "engagement" or clicks and likes that link back to Facebook advertising. By 2014, Facebook had restricted the content published from business and brand pages. Adjustments in Facebook algorithms have reduced the audience for non-paying business pages (that have at least 500,000 "Likes") from 16% in 2012 down to 2% in February 2014. 
Digital marketing is defined by the use of numerous digital tactics and channels to connect with customers where they spend much of their time: online. From the website itself to a business's online branding assets -- digital advertising, email marketing, online brochures, and beyond -- there's a spectrum of tactics that fall under the umbrella of "digital marketing."