Consider MySpace’s efforts to scan its entire user base of 480 million user accounts for RSO’s:

• What criteria should be used to make a positive match on a RSO? Name match? DOB? If the person looks like a known RSO?
• Should the legal definition of RSO be used? If so, you will need to include teenagers who have (gasp) had sex with each other.
• Like any human process, errors will be made. What is an acceptable false-negative rate? 1%? 10%? How is a social network to measure such a metric?

Cashmore nicely sums-up the outcome of such well-intentioned, but often flawed initiatives:

…punishing rapists and sex offenders may feel satisfying, but this isn’t what the bill does:it’ll likely affect those who committed far lesser crimes, prove unenforceable, and may even be unconstitutional.

via Sex Offenders Banned from Social Media Sites.