“It’s one of the secrets we don’t let a lot of people know,” said Phil Landry, a probation officer who works out of Quincy District Court and uses his personal iPad on the job.

Today, more than 81,000 people in Massachusetts are on probation, and social media and other computer technology are helping the state’s 809 probation officers supervise them in ways that were never possible before.

Officers can look on social networking sites to see if people on probation are associating with gang members, drinking, using drugs, or doing anything else that might violate the conditions of their probation.

Kade Crockford, director of the ACLU of Massachusetts Technology for Liberty Project, said there is nothing to stop officers from “trolling the accounts” of social media users, but she doesn’t think it’s appropriate for officers to create fake profiles to “go undercover to snoop on their charges.”

Probation officers should also exercise caution on how they use information that they glean from social media sites, according to Peter Ubertaccio, a political science professor at Stonehill College.

“Sometimes musings on ­Facebook are just that,” said Ubertaccio. “People can post anything.”

A person might write a tantalizing status update or post a photo of themselves at a raging party, but in reality, they are “pretending or just ranting online, when they’re really safe at home,” he said.

But when postings are accurate, they can lead to useful information for law enforcement and corrections officials.

Ann MacDonald, a Plymouth Juvenile Court probation officer, once found the Facebook profile of a man who’d had an outstanding warrant for 15 years.

MacDonald also used Facebook to track down a Middleborough teenager who had run away with a circus. The 16-year-old was on probation for a larceny charge when he disappeared in July.

MacDonald cannot access Facebook in her Wareham office because the courthouse computer network limits Internet access and blocks certain sites. So she used her home computer to try to locate the wayward teenager, logging on to Facebook in the evenings to look for clues about where he was and what he was doing.

From the photos he was posting publicly, she soon confirmed that he was working as a “carnie” and traveling to different states.

She also noticed a pattern. “Wherever the circus was, he would add a friend on Facebook” who lived in that area, she said.

She located the circus company’s website, checked its tour schedule, and learned that the next stop would be in Waverly, Va. MacDonald contacted the State Police in Virginia at 4:40 p.m. and sent them a picture of the youth. Police went to the circus and arrested him that day.

“By 6:30 they had the kid,” said MacDonald. “It was a pretty amazing thing.”

The boy was returned to Massachusetts on Oct. 1.

MacDonald views social media as “one more tool in the toolbox” to find out what’s really happening in the lives of teens. “We’re able to see a different side,” she said.

Before it was MySpace. Now it’s Facebook and Twitter and Instagram.

“Juveniles have a tendency not to be bright about what they post,” on those sites, said MacDonald. “Many times I’ve gotten kids posting pictures of themselves using drugs, pictures of themselves with alcohol. . . . Kids are tweeting all the time, ‘I just got high,’ ” she said.

Still, she is astonished by what teenagers post about themselves online. “Even though they know you have access, they still post things that are inappropriate,” she said. “They think they’re not going to get caught.”

Such brazen posting is familiar to Plymouth District Court Probation Officer James Polin, who also brings his personal iPad with him to work.

This past January, Polin was talking with a 25-year-old Plymouth man at the courthouse who was on probation for possession of Class B drugs. The man, who had just failed a mandatory drug test, suddenly ran out of the courthouse and escaped.

After a warrant was issued for his arrest, Polin kept an eye on the man’s Facebook page, and was surprised to find some things that gave officers an idea of where he was.