How can I monitor only one thread (post and the connecting comments)?

You need to create a new Brand or Advanced project and try one of the following approaches:

In the Keywords text box, insert a part of the text of the post in quotation marks. This search is contextual: you will get both the post and its comments.

Build a project for a specific Facebook post and its comments, as well as for online articles and their related comments. You can search for a post by putting a unique part of its text into the advanced keywords search between quotation marks, and if you search the contextual mode, you’ll also get the related comments.

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Don't forget

You cannot build a project for a single Tweet and it’s replies. This is because of the limitations in the Twitter API.