Perhaps Facebook users have seen zit burst by noon (8)

'perhaps facebook users' is the definition.
(netizen can mean an internet user)

'seen zit burst by noon' is the wordplay.
'burst' is an anagram indicator.
'by' means one lot of letters go next to another.
'noon' becomes 'n'.
'seen'+'zit'='seenzit'
'seenzit' with letters rearranged gives 'etizens'.
'etizens' after 'n' is 'NETIZENS'.

'have' acts as a link.