You can create a page without any playable builds, to start promoting your project. You can even accept pre-orders if you’d like. Just make sure to mark your game accordingly in the “Release Status”.
One thing to keep in mind is, your project will not be indexed in the search engine until it gets a playable build. Your page however will still be public and you can share it’s URL with other social media.
Can I share my social media page like instagram or my twitch.tv channel?
As far as I’m aware, you can add your own social media links on the project’s page. Although not sure if it’s a rule, I’d imagine it gets into a gray area if you decide to promote competitor marketplaces. If all you want to share is social media where you document your progress, or for people to follow you, that should be fine.
It’s up to you to make sure potential viewers are enjoying your page and are not overwhelmed by external links.
Hope that helps