  Cybercriminals abuse Online Social Networks (OSNs) to lure victims into a
variety of spam. Among different spam types, a less explored area is OSN abuse
that leverages the telephony channel to defraud users. Phone numbers are
advertized via OSNs, and users are tricked into calling these numbers. To
expand the reach of such scam / spam campaigns, phone numbers are advertised
across multiple platforms like Facebook, Twitter, GooglePlus, Flickr, and
YouTube. In this paper, we present the first data-driven characterization of
cross-platform campaigns that use multiple OSN platforms to reach their victims
and use phone numbers for monetization.
  We collect 23M posts containing 1.8M unique phone numbers from Twitter,
Facebook, GooglePlus, Youtube, and Flickr over a period of six months.
Clustering these posts helps us identify 202 campaigns operating across the
globe with Indonesia, United States, India, and United Arab Emirates being the
most prominent originators. We find that even though Indonesian campaigns
generate highest volume (3.2M posts), only 1.6% of the accounts propagating
Indonesian campaigns have been suspended so far. By examining campaigns running
across multiple OSNs, we discover that Twitter detects and suspends 93% more
accounts than Facebook. Therefore, sharing intelligence about abuse-related
user accounts across OSNs can aid in spam detection. According to our dataset,
around 35K victims and 8.8M USD could have been saved if intelligence was
shared across the OSNs. By analyzing phone number based spam campaigns running
on OSNs, we highlight the unexplored variety of phone-based attacks surfacing
on OSNs.
