Extra User Details is the simple plugin that allows you to add extra fields to the user profile page (e.g. Facebook, Twitter, LinkedIn links etc).
Extra fields can be easily accessed in your templates like a general wordpress author details:
<?php the_author_meta('meta_key'); ?>
Plugin saves fields data in wp_usermeta table. You can add and edit extra fields at plugin options section in backend.
This plugin is available for download to be used on your WordPress self-hosted installation.