ReCaptcha is currently working here at UBBCentral, on the New User page; I attempted to test this at your site, but it appears that you've disabled it. I am not seeing any outages listed at:
http://www.isitdownrightnow.com/recaptcha.net.html
http://downdetector.com/status/google
http://www.websitedownup.com/recaptcha.net

As for the built in libraries, as long as the captcha image displays in the Control Panel selection, it should display for your users (as it utilizes the same coding).

Make sure that your users aren't blocking Javascript, or see if they're using a security plugin that'd block 3rd party (offsite) scripts.

I am a Web Development Contractor, I do not work for UBBCentral. I have provided free User to User Support since the beginning of these support forums.
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