What is Link Building?
Link building is one of the oldest and best practices in search engine optimization (SEO). When a website contains your site’s link (i.e. links to your site), in terms of search engine score, it’s a plus point for your site. The generic term you’re probably looking for is ‘backlink’.
PageRank was invented by Google’s co-founder and current CEO, Larry Page, and it’s one of the determining factors in where pages rank in Google. Google views links to other websites as votes of confidence that the website has quality content. It’s not entirely democratic. Pages that have been deemed important by their higher PageRank in turn exert more influence with their links. This transfer of importance is also called “Google juice.”
Sometimes you don’t want to add additional Google Juice to Affiliate or marketing link. There’s an easy solution. Use the nofollow tag. Google won’t follow the link, and you’ll remain in good standing with the search engine.
The important thing to remember is that not every site is the same. In some situations.. maybe a single landing page where all links are affiliate or a site where all outbound links are downloads you might want all the links to be nofollow. There is one WordPress Plugin called “External Links” only changes links in the post content. You can always have external links elsewhere. One important point to note here is that: WordPress adds automatically adds nofollow to all links in comment sections.
NOTE: Adding nofollow to all links is definitely not something for everyone and it’s something I wouldn’t recommend a blogger, but in some cases it can be very useful.
Should I add rel=”nofollow” to all Social Share links?
Yes, you should. Let’s check it out. I’m using simple social sharing buttons. 
Please right click on any of the icon and click inspect element (if you are on Chrome browser). You should see something like this. 
These endpoints are intended to be called directly by browsers and are designed for user interaction, and therefore cannot be called by scripts or backend services. We strongly recommend adding rel="nofollow"attributes to your HTML tags, otherwise search engine crawlers who follow your sharing api links can generate false shares in your analytics reports.
Sample Example:
<a href="http://examplesite.com" rel="nofollow">Your anchor text here.</a>
How to Add NoFollow Attributes to all Links in a Specific Category?
If you want to add nofollow to all links in a Specific Category then simply put below code to your functions.php file.
function crunchify_nofollow_cat($text) {
global $post;
if( in_category(1) ) { // YOUR CATEGORY ID HERE
$text = stripslashes(wp_rel_nofollow($text));
}
return $text;
}
add_filter('the_content', 'crunchify_nofollow_cat');
You may also want to check out list of all WordPress Tips.