holy spam batman!

[ZiNgA BuRgA]

SFS blocks legit users too.  The problem with the IP matching it uses is that some spammers are going through dynamic IPs, which can be assigned to legit users as well.
Also, SFS have been known to ban entire email domains.

reCaptcha is good, but from my trial run of my own CAPTCHA solution, it doesn't appear to really block much.  Either bots are able to bypass it (not that likely) or they automatically detect and send it to crowdsourcing (much more likely); in either case, it isn't terribly effective at stopping spam registrations.
I use a lesser known CAPTCHA service here, which seems to stop all the automated attempts.