This can be set for each individual address book. If toggled on, it mean contacts will be added to your My Contacts list, as well as combined with any other connected sources. These contacts will sync back to your connected sources.
If toggled off, you are essentially making that connected source a part of your FullContact account, but those contacts will only be used to update existing Contacts in your FullContact account. In other words, they are kept separate from My Contacts (no changes will be written to-and-from your My Contacts).
You can manage your sources from the Settings page.
Furthermore, for LinkedIn, we recommend setting these to Update Existing Contacts.
This means that we are ONLY updating the contacts that already exist in your My Contacts. No new contacts are added to your My Contacts; however, all contacts from a source set to Update Existing will still be added to their individual address book. You can view all contacts within an individual address book here:
Alternatively, you can choose Add as New Contacts. This will do two things:
1. It will add contacts from this source as new contacts to your My Contacts.
2. It will simultaneously update existing contacts with new information.
*Keep in mind that choosing Add as New Contacts will create new contacts in your My Contacts without any useful information (i.e. a contact with only a Twitter handle).
For an imported CSV or vCard file, we recommend choosing Add as New Contacts. For example, if you currently have your work Google Contacts syncing with your My Contacts in FullContact, any contacts from your CSV or vCard imported file will not only be added to your Google Contacts as new contacts, but we'll simultaneously update existing contacts (with data from the import) as well.
Lastly, your Twitter, Foursquare, and Xing address books will all be set to Update Existing Contacts.