If you are running campaigns to re-active users who have downloaded a mobile application, the only way to measure results is through Facebook's SDK. The conversion pixel is used for websites.

You want to use CPM for brand awareness campaigns.For direct response campaigns, you want to use CPA or optimize for conversions, as you are interested in getting people to use the app.

You also want to market users who HAVE downloaded the app. To build this audience, you need a custom audience. Core audiences are for reaching out to new markets.