It seems Facebook have a new endpoint to share links https://www.facebook.com/share_channel/, with the following query parameters: type=reshare, link=[URL ENCODED LINK], source_surface=external_reshare, display and hashtag

Which gives you the following:

https://www.facebook.com/share_channel/?type=reshare&link=https%3A%2F%2Fgoogle.com%2F%3Futm_source%3Dfacebook&app_id=x&source_surface=external_reshare&display&hashtag