"Linezolid is an antibiotic with myriad uses worldwide, including treatment of pneumonia and skin infections."
No, I think myriad is not correctly used here. When used as an adjective it describes something of such a great number as to be uncountable; but, a little research into linezolid indicates that its uses can be broadly categorized into a few categories of bacterial infection control. Should the writer wish to convey that he or she is speaking of the skin infections or pneumonia within such a broad category, it is still not "innumerable" because it is possible to count them.
There is an article on the syntax involved in Fowler's Dictionary of Modern English Usage By Henry Watson Fowler, Jeremy Butterfield (reformatted):
Rather inexplicably, myriad has become a usage issue for certain people. The word is quite legitimately, and with good historical precedent, used in three [four?] syntactic patterns (dates of first known use [given] in the OED in brackets):
(i) As a plural noun [+ of]: myriads of stars (1555)
(ii) As a singular noun + of: a myriad of stars (1609)
(iii) As a … quantifier: a myriad stars; myriad stars (1735)
… The first usage book to proscribe [the noun usages] dates from as recently as 1996.
The usage in the above example would thus be considered not ungrammatical by Fowler.
As regards meaning, Fowler goes on to say:
[T]o use it when the numbers involved are not large cheapens it; [but] often it seems to mean little more than ‘a large range’.
So Fowler would not recommend the use of myriad here on style grounds (unless there are say thousands of uses of Linezolid).