I think by now, most of the principles of SEO are more or less well understood by people that have done their time in the trenches since 1999, although new platforms and data types arrive all the time keeping the need for ongoing practice or testing even more important than in the past. The need to learn about new entrants like Siri, Alexa, Facebook, Linkedin, and specialist search engines (like Booking.com, for example) will keep SEOs on their toes for years to come.

Often the line between pay per click advertising and paid inclusion is debatable. Some have lobbied for any paid listings to be labeled as an advertisement, while defenders insist they are not actually ads since the webmasters do not control the content of the listing, its ranking, or even whether it is shown to any users. Another advantage of paid inclusion is that it allows site owners to specify particular schedules for crawling pages. In the general case, one has no control as to when their page will be crawled or added to a search engine index. Paid inclusion proves to be particularly useful for cases where pages are dynamically generated and frequently modified.
Social media often feeds into the discovery of new content such as news stories, and “discovery” is a search activity. Social media can also help build links that in turn support into SEO efforts. Many people also perform searches at social media sites to find social media content. Social connections may also impact the relevancy of some search results, either within a social media network or at a ‘mainstream’ search engine.

Page and Brin founded Google in 1998.[23] Google attracted a loyal following among the growing number of Internet users, who liked its simple design.[24] Off-page factors (such as PageRank and hyperlink analysis) were considered as well as on-page factors (such as keyword frequency, meta tags, headings, links and site structure) to enable Google to avoid the kind of manipulation seen in search engines that only considered on-page factors for their rankings. Although PageRank was more difficult to game, webmasters had already developed link building tools and schemes to influence the Inktomi search engine, and these methods proved similarly applicable to gaming PageRank. Many sites focused on exchanging, buying, and selling links, often on a massive scale. Some of these schemes, or link farms, involved the creation of thousands of sites for the sole purpose of link spamming.[25]
However, while bidding $1,000 on every keyword and ranking #1 for every relevant search sounds nice in theory, most businesses have to play a balancing game between ranking higher and paying too much for clicks. After all, if it costs $17.56 to rank in position #1, but you can only afford to pay $5.00 per click, bidding $1,000 on a keyword to guarantee yourself the #1 position would be a great way to bid yourself out of business.
Back end tools, including Web analytic tools and HTML validators, provide data on a website and its visitors and allow the success of a website to be measured. They range from simple traffic counters to tools that work with log files and to more sophisticated tools that are based on page tagging (putting JavaScript or an image on a page to track actions). These tools can deliver conversion-related information. There are three major tools used by EBSCO: (a) log file analyzing tool: WebTrends by NetiQ; (b) tag-based analytic tool: WebSideStory's Hitbox; and (c) transaction-based tool: TeaLeaf RealiTea. Validators check the invisible parts of websites, highlighting potential problems and many usability issues and ensuring websites meet W3C code standards. Try to use more than one HTML validator or spider simulator because each one tests, highlights, and reports on slightly different aspects of your website.
As we’ve been told by the Google rater’s guide, we ideally want to focus upon building sites that raters might see as involving Expertise, Authoritativeness, and Trustworthiness. A recent Google patent told us that it might look at search results to gauge the authoritativeness of pages in search results, and if pages within those results don’t meet a specific threshold of authoritativeness, Google may perform a search on a related query to find other authoritative search results, which could then be blended into search results for that initial query.
In the social sphere, things change fast. New networks emerge, while others go through significant demographic shifts. Your business will go through periods of change as well. All of this means that your social media strategy should be a living document that you look at regularly and adjust as needed. Refer to it often to keep you on track, but don’t be afraid to make changes so that it better reflects new goals, tools, or plans.
In the 2000s, with more and more Internet users and the birth of iPhone, customers started searching products and making decisions about their needs online first, instead of consulting a salesperson, which created a new problem for the marketing department of a company. In addition, a survey in 2000 in the United Kingdom found that most retailers had not registered their own domain address.[12] These problems made marketers find the digital ways for market development.
Creating high quality content takes a significant amount of at least one of the following: time, effort, expertise, and talent/skill. Content should be factually accurate, clearly written, and comprehensive. So, for example, if you describe your page as a recipe, provide a complete recipe that is easy to follow, rather than just a set of ingredients or a basic description of the dish.
Inbound marketing refers to a marketing methodology wherein you attract, engage, and delight customers at every stage of the buyer's journey. You can use every digital marketing tactic listed above, throughout an inbound marketing strategy, to create a customer experience that works with the customer, not against them. Here are some classic examples of inbound marketing versus traditional marketing: 
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