FB no longer allows pre-fill.

info.plist

  <key>LSApplicationQueriesSchemes</key>
	<array>
		<string>fb</string>
	</array>

UIActivityViewController∓Facebook.swift

extension NSString:UIActivityItemSource {
  public func activityViewControllerPlaceholderItem(_ activityViewController: UIActivityViewController) -> Any {
    return ""
  }
  
  public func activityViewController(_ activityViewController: UIActivityViewController, itemForActivityType activityType: UIActivityType) -> Any? {
    let installedFB = UIApplication.shared.canOpenURL(URL(string: "fb://")!)
    let isFB = activityType == UIActivityType.postToFacebook
    if installedFB && isFB {
      return nil
    }
    
    return self
  }
}