Getting hacked on Twitter is fast becoming a rite of passage for big corporations, but Tuesday's attack on the Associated Press could be a tipping point and shows that social networks must do more to keep their users safe, security experts said. Wider use of two-factor authentication, which can involve an access code being sent to a user on a second device such as a smartphone, is one possible solution. Such a mechanism could be introduced selectively, some experts said, for high profile accounts such as celebrities and large corporations.