Job Recruiters asking for Last 4 of SSN. Scam?

How many people looking for a job lately have gotten very pushy calls from supposed Job Recruiters wanting your last 4 digits of Social Security Number, and even birthdays ?

I've concluded there is a huge Identity Theft ring(s) using this technique very successfully. They will spend huge amounts of time and energy (emails, calls) to try and talk you into giving it up. What about it? Anybody want to share experiences? After researching this, I ended up joining LifeLock today, just in case!