FBSDKShareLinkContent *content = [[FBSDKShareLinkContent alloc] init];
content.contentURL = [NSURL URLWithString:APPSTORE_URL_MAIN];
content.contentTitle = INVITE_TITLE;
content.imageURL = [NSURL URLWithString:INVITE_IMG_URL];
content.contentDescription = INVITE_TEXT;
[FBSDKMessageDialog showWithContent:content delegate:self];
If the Facebook Messenger app is closed, this just opens the app and does nothing else.
Works fine is the FB Messenger app is working in background
Issue resolved by Facebook Developers.