Thred Launch, page-13

Thred imports all of your contacts from everywhere into your master address book within Thred. All you do is download the app and link your platforms through it. This includes all contacts in your phone; email; Google; Twitter and Facebook as at time of V1 Beta testing.

Your 200 FB friends do not automatically receive an invitation to join Thred just because you have linked platforms and your contacts have been imported.

You can search for individual invitees by contact name or platform for those you wish to involve in a Thred. The invitation to join your Thred will be sent to the contact on the platform of origination. For example in the "Messages" tab in Twitter and/or as a text with attached link via SMS. The invitee then clicks that link to join the conversation and upon signing up downloads the Thred App and the process begins again with contact imports and platform linking.

I did not see the FB part in action. Presumably a notification appears in messenger similar to appearing within your private messages on the Twitter platform.
Standard invite starts as "You have been invited to a Thred. Head over to (Link) to get started"