Its important that you setup your social channels and interlink them and then engage with your users on social with the right content and drive traffic to your site through these channels. Racking up fake signals and fake followers who do not engage or visit your site through the channels, is easily detected by Google as false and it does not help your rankings.
It depends on what you need to do… If you just need to lookup search volumes, then KWFinder is a winner and cheap. Check out my other blog post on SEMrush that describes the 6 different ways I use it: If you don’t need any of those features, then I would go with KWFinder. If you need some of those features, then I would go with SEMrush as you can lookup search volume in both.
They also seem to be getting this wrong often enough that I've got less confidence that the keywords that make up these groups really belong there. I recently tried to check the volume for the keyword [active monitoring] (the practice of checking on a network by injecting test traffic and seeing how it's handled, as opposed to passive monitoring) and the Keyword Planner gave me the volume for [activity monitor] (aka Fitbit).
When you go about doing your content marketing and spreading your content online, you must post the right content in the appropriate social channel - for example video on Youtube about new product line vs business anniversary on Twitter or Linkedin. This also ensures that people engage at higher levels with your content and are not spammed with content they do not want.
You can find broken internal links from within the Search Console. You need to attend to each warning appropriately telling Google that you have fixed it. Having excessive 404s will hurt your site if they are really 404s, because anyone could escalate the 404s by pointing randomly to pages that don't exist from external places, which is why this is not that big of a deal - but should be looked at.
Jaaxy is an online keyword finder owned by Kyle Loudoun and Carson Lim that promises to help you find low-competition keywords that will help you improve your rank in the search engines. Other Jaaxy features include alphabet soup, which allows you to brainstorm for keywords; saved list, which allows you to save your list of keywords so that you can view them later; and search analysis, which lets you search what is already on search engines such as Yahoo, Google, and Bing. Jaaxy offers a free trial as you get started, and you can also choose between the pro version and the enterprise version if you like how it works.
Of all the tools listed in this article, Moz Link explorer is an old one & quite popular.  If you want to compare backlinks between two or more domains, Open Site Explorer is worth trying. This tool works best when you have a paid account of SEOMOZ though a free version of this tool is good enough to get you started checking the backlinks of your site and the sites of your competitors.