They certainly are vulnerable. The solutions for social networks are in client/behaviour analysis: Are you trying to create 10th account from the same IP? Are you creating multiple accounts with the same browser fingerprint? Have you got any personal details attached? (That's one reason they started pushing phone verification) Is the action automated? (CAPTCHA) Is your friend graph a clique of fresh accounts?
A lot of these can't be applied to SKS unfortunately.
A lot of these can't be applied to SKS unfortunately.