I created an Instagram feed yesterday just to run ads.
I also have a Facebook page for two apps. Unfortunately I'm quite lazy and don't post much.
1000 likes so far for this one: https://www.facebook.com/PackrApp/
Currently what brings the most people to my App Store pages are:
I had a go at App Store Search Ads but the ROI isn't interesting at the moment.
For #3, in each of my apps I have a screen that displays all of my other apps. Also random pop-ups linking to my other apps.
This works really well for one of them, approximately 20% of all revenue is generated by that specific screen in the other apps.