Overwiew
Because JomSocial is component for Joomla, and it does not have its own user object, many users can't figure out why sometimes registration and login form lead to native Joomla forms for this task.
Some Template clubs override the native Joomla login form which by default, should be accessible with this link
http://yourdomain.name/index.php?option=com_users&view=login
If you visit your site by using this link, in most cases you will be presented with the following screen
If you login using this form, you will most likely see something like this
If you click on the registration link, you will be redirected to native Joomla registration form
This is intended behavior, because Joomla as an user-based CMS, designed to work without JomSocial installed, and after all, we are currently on the com_users component, which is not JomSocial, and have nothing to do with it.
JomSocial Redirect Plugin
Since not many users do not know or do not want to bother with minor customization and hacking, we provided the [Plugin] which should redirect all registration requests to JomSocial's form.
This plugin however, won't do anything if user use Joomla login form to access the site. It will still be presented with native Joomla user page.