When you first start, growth can quickly become an all-consuming goal. While numbers are great, for a meaningful and quality experience, you'll have to do things that don't scale. You need to share content regularly, connect 1-on-1 with readers and community members, etc. I suggest you go all-in on one social platform instead of trying to appear everywhere. Focus on getting results in one place before adding more to your plate.
Facebook, Twitter, and Instagram are the top choices, but LinkedIn (business people) and Pinterest (women's interest) can be suitable depending on the book. TikTok is the place to be for YA right now.
If your accounts are not established on social media at least 6-12 months prior to book launch, they will likely play a minimal role for that book’s sales. Do not join social media just to promote your book; do it for long-term relationship and platform building.
Should you hire a social media assistant? I don't endorse this approach, but some people feel they must hire, so if that describes you: https://www.janefriedman.com/should-you-hire-a-social-media-assistant/
And, YES, you can sell your book without social media, in case you were wondering: https://mailchi.mp/hotsheetpub/rwa-future#mctoc4