There's a common misconception that posting on social media helps increase your website's rank on search engines. As Moss Clement states in our guide to 7 Ways Social Media Can Improve Your SEO Results:

Social media does not directly contribute to SEO ranking, but the links you share across social platforms increase brand exposure.
Your shares across social media sites have no place in SEO rankings. Still, when more people share your content throughout social media, it generates social signals that indicate your posts are useful to your target market.

While social media doesn't directly increase your search engine rankings, it indirectly affects and helps build your brand. 

You've probably heard that links pass PageRank. But what exactly is this?

PageRank is "a system for ranking web pages that Google's founders Larry Page and Sergey Brin developed at Stanford University. And what it is important to understand is that PageRank is all about links."

We can think of the PageRank of a web page as a vote of authority, and this authority can then be passed to other pages or websites through links (both internal and external). In simple terms, it's a measure of the importance of a web page according to the quantity and quality of links that point to it. 

NO, NO, NO! 

Buying links is a substantial violation of Google's Webmaster Guidelines and could result in your website being penalized. The link scheme guidelines clearly state that the following links are examples of those that can negatively impact a site's ranking in search results:

Buying or selling links that pass PageRank. This includes exchanging money for links, or posts that contain links; exchanging goods or services for links; or sending someone a “free” product in exchange for them writing about it and including a link.

Links should never be bought, only earned.