Default Sharing Dialog

As an alternative, you can show a standard ios sharing dialogue box and let a user decide where to post by his own. The sharing dialogue will look similar to the one on a screenshot below:

The implementation approach is similar.

Sharing a text message

IOSSocialManager.Instance.ShareMedia("Some text to share");

Or text message with an image

Texture2D image = GetImage();
IOSSocialManager.Instance.ShareMedia("Some text to share", image);