Good morning,
For these job I would create 2 storytelling: one for the Users and other for the Subscribers but both would Interact between then.
Instagram: photos and explanation on a really conceptual way to show your app. Telling from the pictures how users and subscribers can use you app, and why they should download that.
Facebook: 2 different types of advertising, coughing the right public, choosing interest profile, and sending straight to the right people.
Pinterest: Banners with call to action with the app link on the description, sending people straight to download.
For social media, frequency is the key of gaols achievement, so I would create a calendar with all the actions for each social media, and the days people will receive the content in their feed. After a month we go back to the calendar and see de RESULTS for each post. And valuate with we need to change the strategy or should we keep with that.