The answer is: they did nothing. They all new it was phoney.
Right. The sole objective of the dossier and the secret FISA warrant it spawned, was to create a "legal" excuse to collect electronic intelligence on Trump and his campaign. Full surveillance on Page enables the option of full surveillance on his contacts. He was obviously at various times in contact with Trump and members of his campaign staff.
The FBI leadership had no interest in surveilling Page himself - in fact he was probably working UC for them.