Hi Giacomo,
There’s nothing step-by-step for Facebook, but there are some sections on this blog post that you might find useful on working with REST APIs in KNIME.
If Facebook APIs use tokens, here’s a forum post that shows how to handle tokens for Google APIs in KNIME that you could use a template as well. Having difficulties setting up Post Request to Google API requiring OAuth2.0 - #13 by alabamian2
Edit: Feel free to send any errors or difficulties you have in this thread and maybe we can help 
Regards,
Wali Khan