Question

I’m now working on a web application which is mostly based of facebook graph api.
I hold some data about users – actually , the possible public data available – such as name and id.
I also know that a profile picture is a part of the public data, and I wondered how I’ll be able to get a direct link to a profile picture of a user only by using their id?

Answer 1

http://graph.facebook.com/" + facebookId + "/picture?type=square
For instance:
http://graph.facebook.com/67563683055/picture?type=square

There are also more sizes besides "square". See the docs.

Update September 2020
As Facebook have updated their docs this method is not working anymore without a token. You need to append some kind of access_token. You can find further information and how to do it correctly in the fb docs to the graph api of user picture

Requirements Change
This endpoint supports App-Scoped User IDs (ASID), User IDs (UID), and Page-Scoped User IDs (PSID). Currently you can query ASIDs and UIDs with no requirements. However, beginning October 24, 2020, an access token will be required for all UID-based queries. If you query a UID and thus must include a token:

use a User access token for Facebook Login authenticated requests
use a Page access token for page-scoped requests
use an App access token for server-side requests
use a Client access token for mobile or web client-side requests

Quote of fb docs