WASHINGTON — Maxi Sopo was having so much fun "living in paradise" in Mexico that he posted about it on Facebook so all his friends could follow his adventures. Others were watching, too: A federal prosecutor in Seattle, where Sopo was wanted on bank fraud charges.

Tracking Sopo through his public "friends" list, the prosecutor found his address and had Mexican authorities arrest him. Instead of sipping pina coladas, Sopo is awaiting extradition to the U.S.

Sopo learned the hard way: The Feds are on Facebook. And MySpace, LinkedIn and Twitter, too.

Law enforcement agents are following the rest of the Internet world into popular social-networking services, even going undercover with false online profiles to communicate with suspects and gather private information, according to an internal Justice Department document that surfaced in a lawsuit.

The document shows that U.S. agents are logging on surreptitiously to exchange messages with suspects, identify a target's friends or relatives and browse private information such as postings, personal photographs and video clips.

Among the purposes: Investigators can check suspects' alibis by comparing stories told to police with tweets sent at the same time about their whereabouts. Online photos from a suspicious spending spree – people posing with jewelry, guns or fancy cars – can link suspects or their friends to crime.