Facebook – How many friends I can tag on the Facebook photo

Simply preface a person's name with an @ sign when you type it in a status. For instance, if you want to tag your friend Joe Someone in your status, begin by typing @Joe. Facebook will automatically open a drop down where you can select his name.

This is accomplished through the Facebook Graph API.

At Facebook's core is the social graph; people and the connections they have to everything they care about. The Graph API presents a simple, consistent view of the Facebook social graph, uniformly representing objects in the graph (e.g., people, photos, events, and pages) and the connections between them (e.g., friend relationships, shared content, and photo tags).

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The connections supported for people and pages include:
Friends: https://graph.facebook.com/me/friends?access_token=...
Photo Tags: https://graph.facebook.com/me/photos?access_token=...
Photo Albums: https://graph.facebook.com/me/albums?access_token=...
Video Tags: https://graph.facebook.com/me/videos?access_token=...

…among others.