In summary: no user data, personal or otherwise, is transmitted outside of the extension.
By initiating a sync of contacts from a supported social network, a list of contacts (consisting of each profile's ID, URL, and name) is saved in the extension's local storage, managed by your browser. No other data about the contacts, or the associated social network account, is stored.
By selecting an address book CSV file to upload, a list of contacts (consisting of each contact's name and email address) is similarly saved by the extension. All other fields (except the name and email columns you select) are ignored.
Searching for contacts is done offline and locally using this stored data. Stored contacts can be removed at any time.