Port Talbot Town FC midfielder Daniel Thomas posted a homophobic message on Twitter, which was later distributed more widely, leading to his arrest, but Mr Starmer said the message was "not so grossly offensive that criminal charges need to be brought".
"It was not intended to reach Mr Daley or Mr Waterfield, it was not part of a campaign, it was not intended to incite others and Mr Thomas removed it reasonably swiftly and has expressed remorse."
New guidelines for prosecutors are needed because this was one of a growing number of cases and there are likely to be many more, he said.
Within hours, Greater Manchester Police announced the arrest of a 22-year-old man after a Facebook page was set up lauding suspected police double killer Dale Cregan as a "legend".
Mr Starmer said: "To ensure that CPS (Crown Prosecution Service) decision-making in these difficult cases is clear and consistent, I intend to issue guidelines on social media cases for prosecutors."