Other apps connecting to Facebook

Hello. Some of my other apps on my phone allow me to connect to Facebook. For example EventimUK! When I click on connect with Facebook, it says would you like to sign in with Facebook, so I click yes, and then when it opens up it says download Facebook for iOS, which is strange as I already have it on my phone. When I have done this before, it used to just open to Facebook immediately and then go back to original app!