RS have increasingly incorporated
social information (e.g., trusted and untrusted users, followed
and followers, friends lists, posts, blogs, and tags). This
new contextual information [145,216] improves the RS. Social
information improves the sparsity problem inherent in memorybased
RS because social information reinforces traditional memory-
based information (users ratings): users connected by a network
of trust exhibit significantly higher similarity on items and
meta-data that non-connected users [132].