Security in Android apps

Hi,
I am Conrad

I was wondering, if anybody would be able to hack accounts on Android apps.
I mean for example we are logged on ebay or Facebook app, and we simply quit to home screen, without logging out, and also disconnect from network and again turn on network.
-To the point, Is it technically possible if anybody would hack our session, when we logged?
Of course we're using WiFi in home with WPA2, not public hotspot.

  • How does situation look like with bank apps? Is it any risk when I immediately leave the app without logging out,
    -But in web browser on android, on Facebook, is it possible to take over session?

-I 've found VPN app "Zenmate" on Google Play is that proper VPN? , is it worth to use it? or that is a piece of junk?