MailChimp now offers landing page functionality.
You can promote a landing page via social media posts, for example.
Give people good reasons to join your list, when they arrive at the landing page.
You can choose between two landing page templates, within MailChimp.
They are mobile responsive.
I found it to be very easy to create my first landing page as the look and feel is essentially the same as the Campaign creation process:
If you need to edit your MailChimp landing page, you will need to unpublish it first.
If you are interested in MailChimp training click here.