Platforms are ways to reach an audience. The way to reach a wider readership audience is through the readership you already have, and their recommendations. But to reach the readership, you need another kind of platform, a set of social media tools that you are using in the best way.
I think that the most flexible and powerful social platform is one you control, your own website. Sure, Facebook, Twitter, Instagram, Pinterest, and all the others can be important, but any of them can disappear. Worse, the content that you’ve carefully added to accounts on them can disappear without recourse.
Registering your name as a domain is the first step, and it is still cheap if you can get the ‘.com’ domain. Other endings (technically called top level domains) are possible, and the ‘.author’ ending has been approved by the gods of the Internet but is not yet available for use due to wrangling over who controls it. While .com addresses are cheap, .author addresses will probably be sold for a premium.
My recommendation is to get YourName.com if you can. Modify this advice if you live outside the US and getting YourName.uk or YourName.cz (country specific addresses) is more important to connecting with your readers.
The other message here is that you are building a platform for you, the author. Not your first book, not your current book, you. Those things might get their own sites, or they might have space on your site, but the platform is about you, a person, connecting with your readership, other people. Unless you are a dog.
Next up, why WordPress is such a great choice for your platform platform.