According to leaked Snowden documents, American and British intelligence agents have virtually infiltrated World of Warcraft and Second Life. Based on reports from The Guardian and The New York Times, documents dating back to 2008 point at agents creating anonymous characters inside of both game worlds in order to keep tabs on fake gamers.
Apparently the reasoning is that nefarious individuals and even those involved in possible terrorist activities could be using the anonymity of these MMOs to secretly communicate, plan attacks and even transfer funds. The documents specifically outline FBI and CIA agents actively "infiltrating" Second Life with something called a "deconfliction" group put in place, in order for the agents to know who their fellow virtual spies were.
In addition to the WoW and Second Life spying, these group also used the UK's GCHQ's open-source packet-sniffing software to keep an eye on communications over these networks. According to the leaked documents, these logs have since been "forwarded to GCHQ for additional analysis, target development, and network knowledge enrichment."
The documents go on to say that the GCHQ has discovered possible signal intercept operations by "identifying accounts, characters, and guilds related to Islamic extremist groups, nuclear proliferation, and arms dealing." Beyond that, it looks as though Xbox Live gamers have also been spied on, both the NYTimes and The Guardian report that a GCHQ document claims that agents have successfully intercepted "discussions between different game players on Xbox Live."
It is important to note that the documents at hand are five years old, so it is unclear how deep government agencies are still involved in these gaming networks. Apparently the agencies have been very aware of the boom in mobile gaming since before Apple's App Store made its debut as well, so its likely the mobile space has become a target since then.