Hey @evamelba I did a test and sent the link to myself as a private message via facebook and opened it and it went to the correct page, so you did everything right! 
In the social preview, it will typically just take the root domain and display it, along with the OG (open graph) description & image uploaded to the website (but in actuality it will link to the page the button is on as that is how the js works).
Here is a link about adding OG (Open Graph) images: http://forum.webflow.com/t/open-graph-image-not-displaying-correctly/17923?u=waldo
At this time you cannot bind OG description/images per a post, though I’m sure the team is working on this feature so that we’ll be able to set an image/description per a post. So I’d recommend adding an additional field to your Collection for an OG image & description which you can later on bind. 
Lovely site! Please let me know if you have any questions.
Bonne journée!
Waldo