On facebook, I used to search public posts containing specific keywords through R, using the function "searchFacebook" in the package RFacebook, but that function used an API that is discontinued in the new API 2, that is mandatory since April 30.
I gave a look to the APIs and apparently there is no more a method to extract post in this way. Is that true?
Yes, that is true. Public Post Search is gone and there is no alternative, so you can´t search by keyword anymore.
Edit: It is not in the changelog anymore, seems to be too old. Either way, the answer is still correct.