Webservices to create (Register) a new user
Try using the example reference below which displays the php code using OAUTH library with Services module. 

Email verification needs to be done using the rules by adding condition to block user since using oauth it will be activated by default. 

You can use the Facebook/Twitter end-point, consumer credentials(key &amp; secret) replaced to Drupal services.

Reference:
http://www.covenantdesign.com/blog/drupal-services-rest-server-and-3-legged-oauth