Paid Ads on social networks can generate some downloads however engagement for continuous usage of apps is critical to success of your venture. Ads can't buy engagements unless Inapp features are creative or outside app campaigns (instead of ads) are gamified in a indirectly relevant mechanism.

Prelaunch gamification will certainly be very helpful in your case.
I have found that if you reward prospective users of your app using indirectly relevant gamification, users get engaged with your venture. The most activity mobile app user does after texting and phone calls is Take Photos.
I would suggest to create a gamified campaign around taking photos and reward those prospective users for taking 3-5 photos, social sharing etc.
Reward something simpler like what they can physically take home or share or use while traveling etc.
now once the campaign is over, notify all the participants to social share and also introduce your actual app. There will be a great chance of these users becoming your app's genuine users and continue using your product.

We solve these problems with www.oly.io and better gamification under startups market interest discovery www.mobisoftinfotech.com.

Good luck!
Best,
Shail