contacts

 contacts  refers to a set of people, organizations, groups and anything else with one or more addresses (often URLs) for communication.

If you’re looking for contact methods or how to contact someone or a contact page, see:
 * contact

Why
You may want to keep track of contacts for one or more of the following use-cases :

Publish Some Followings

 * "I want to publicly publish a list of people I follow, but there are some people I follow that I do not want public" -aaronpk

Notifications Filter

 * "I want to get push notifications when certain people post, but no notifications for the rest" -aaronpk

Reader Filter

 * "only show me items from NYTimes that 3+ friends have commented on" -tantek
 * "thresholds - only show me items from this person that someone else has commented on (or other activity including liking, reposting)" -tantek

Mention Lookup

 * "What is the URL for John Doe so I can mention him" -benthatmustbeme

Birthday calendars

 * “When do I send someone a birthday card?” -
 * Google Calendar can use Google Contacts to automatically generate a birthday calendar.
 * Facebook automatically generates a birthday calendar of your friends, accessible at https://www.facebook.com/events/birthdays if logged in.

Lookup anytime

 * “How did I get in touch with that person” -
 * Accessible offline and on the go.

Register non-public information

 * “I have the residential address for someone to be able to visit them, I have to store this myself because they do not publish it elsewhere.” -

How to
Contacts may be stored in a local text file, on your server (e.g. like a nicknames-cache), or accessed via a proprietary silo API (e.g. Gmail, Google Contacts), or CardDAV from a silo or some other server.

Exporting your Social Graph from the Silo
Google Data Portability representative: "We want to work on it. We want to do it in a way that is comprehensive but also respects people's privacy. We don't know what the answer is." IWC NYC '18 Data Portability

IndieWeb Examples
(if you have one that others can view, please link directly to it)

Past Examples

 * kept a public view of contacts at https://kylewm.com/contacts (Internet Archive: https://web.archive.org/web/20190405185559/https://kylewm.com/contacts)

InkBlot

 * There is a UI in InkBlot since 2016(?) to allow merging any known identities together (as discovered from webmentions). This allows to have one image for a person no matter which location a reply or like came from. (show the same name / image for a like from twitter and a comment from facebook)