@julian I totally agree with @renchap that it is important to keep the user on the platform of his/her choice! I like the idea of having a better "read more" UX or maybe the lightbox idea.
But to have the best possible experience and to improve the engagement, you should not force the user to leave the platform.
Otherwise it feels very much like subscribing to an RSS-Feed with only excerpts, where you always have to leave the reader for reading the whole text!