Yes, that's right - Moodle accounts could be linked to Project MoodleNet accounts. Apologies if the metaphors didn't work!
To answer your other questions:
Could you expand on what would mean having social networking with other teachers from their Moodle instance?
Project MoodleNet will be a resource-centric social network, meaning that conversation / ratings / tagging is focused on resources. As such, it's valuable to surface this within Moodle instances where teachers and instructional designers spend a lot of their time.
And what do you mean by having better security and reduced costs in Moodle hosting?
I don't know in other countries but normally here teachers also have the role of "instructional designers", and in my case also sysadmin ;)
Thanks for your questions - keep them coming