After a bit effort I've finally managed to get facebook integration working with iOS using Game Maker. Didn't find any solutions online whilst working this out so thought I'd write a quick blog on how I fixed the issue.
To start with I followed this tutorial on how to set things up on the FB side:
Then this one on how to do a simple share:
This resulted in an Android app that worked perfectly and an iOS app that crashed every time I clicked my newly implemented share button and Xcode was giving the following error:
fbauth2 is missing from your info.plist under lsapplicationqueriesschemes and is required for ios 9
So here's how I managed to fix it:
1. Open this file in notepad (directory may vary depending on which version etc.. of GM you're using):
2. Add the following xml just below <dict>:
3. Save the file and close the file, then re-deploy.
Note: You'll have to make this change again if you re-install Game Maker.