There is nowhere near enough information to infer that Mueller obtained a search warrant. If you could read more carefully than have, you would realize that.
First, Facebook has the right to give up information about an account if it so chooses. Second, Facebook only claims that a search warrant is required to compel the disclosure of an account's contents. Disclosing information about ad buys (which, by definition, are not private), and how much was paid (which is Facebook's business as much as the buyer's business), is not the same thing as divulging the contents of an account. Account contents would be non-public posts, messages, pictures, video, like and click information, etc.
There is no question that Mueller is conducting a criminal investigation, so that issuing subpoenas to 3rd parties is neither difficult nor surprising. Such a subpoena could compel Facebook to divulge ad buy info, but not account content.
Personally, I am doubtful that Mueller has probable cause to believe that an ad buy on Facebook is a crime. Why wouldn't a Russian citizen (or government entity) be allowed to buy ads on Facebook?