User login with Social network is auto signing them into my site too?

I have recently installed Events + & I'm liking it so far.
Just sorted the Twitter & Facebook API's out & the logging in to attend courses with them works great.

But should the user then be signed into my site too, as a new member?
I don't know if this is standard?
A bug?
Or something I've done in setting it up wrong?

I haven't as yet used any plugins or API keys on my site that have done this, or gave this access before so I don't know if this is what should be happening?

Obviously by logging in this way they are skipping my registration form!

Thanks in advance,