The huge advantage of email over social media is that prospects and customers are more likely to see an email than social media. Just posting something doesn't mean that everyone you want to see your message will see it. Your post might not even show up in your targets' social media streams. However, an email will sit in the inbox until it's read (or deleted).
InfusionSoft is a fully functional CRM and marketing solution created for small and medium size businesses. Comparing it to MailChimp or AWeber is like comparing a Lamborghini to a VW Golf. It provides highly advanced features for list segmentation, lead scoring and marketing automation. On top of all that, you can integrate all of these features with your optin form using Infusionsoft Tags + OptinMonster.