The US Department of Homeland Security is using social networking sites such as Facebook to check if people are getting into fake marriages for acquiring citizenship.

In a memo dated 13 October, agents of the US Citizenship and Immigration Services (CIS) - an arm of Homeland Security - have been instructed to "friend" those they are investigating.

Befriending them on a social network is expected to grant them access to information they might otherwise not be privy to, when conducting an in-person site visit.

The memo contains steps and online resources that teach the agents how to create a profile and join someone's social network and how to effectively accumulate a large list of friends by joining more networks and sending out numerous friend requests.

The report explains: "Generally, people on these sites speak honestly in their network because all of their friends and family are interacting with them via IM's (Instant Messages), Blogs (Weblog journals), etc."