Most people use social media sites on the Internet to share stories with friends, post pictures of their vacations with family or a night on the town or check up on a long lost friend.

Others use these sites to taunt, harass and bully people who are different from them.

Technology safety expert Katie LeClerc Greer warned an assembly of Vineyard high school students on Wednesday morning that social media sites can present a minefield of trouble if not used properly. Messages posted on Facebook, for example, live on through their digital footprint even when deleted.

“When we put things online and we push that send, share or save button, a permanent record is made,” Mrs. Greer told students. “These cyberbullying cases are tragic and terrible but for law enforcement they are open-and-shut cases because all you have to do is subpoena the [technology] company and everything is there. Everything.”

Dressed in jeans, boots and a sweater, the youthful 27-year-old addressed high school students and later middle school students this week as public schools began the process of adopting a new state-required antibullying policy. The talks were sponsored by the Martha’s Vineyard Youth Task Force. Staff, teacher, student and parent training will continue throughout the school year.

The state mandate follows an incident last year when a 15-year-old South Hadley High School student committed suicide after she was verbally, physically and cyberbullied.

On Wednesday Mrs. Greer underscored the seriousness of using the Internet for casual communications. “It’s really important that once you post something, a picture, video or comment these things stay there and they are recorded, so down the line when you’re applying to colleges or jobs these things could seriously come back and bite you . . . 75 per cent of colleges are now searching Google and Facebook before they admit their students,” she said.

She continued:

“I’m here to tell you today that once you enter into Facebook, you lose all of your rights . . . [The privacy setting] is this luxury they’ve given you . . . [but] it doesn’t mean you have a right to privacy.

“The Facebook terms and services policy . . . says once you enter Facebook you’re entering into a public forum, you then lose the rights to your information.”

Mrs. Greer also spoke to the students about the sensitive subject of child pornography and so-called “sexting” (sending nude or seminude pictures or salacious text messages).

“Child pornography is a felony; that means if you possess it, make it or share it, it is considered a federal offense,” she said, citing an example of two 16-year-olds who sent illicit pictures in a text message. Later one sent a text blast to all his cell phone contacts, forwarding the pictures. Both were charged with possession and distribution of child pornography.

“There are two cases pending in Massachusetts right now with the same exact scenario . . . and in one of these cases they are thinking of charging the entire student body that received this text message,” Mrs. Greer said.

“Technically when these images are sent, whether or not you asked for them, you are in possession of child pornography. If you ever receive an image, whether you asked for them or not, you need to report it immediately to the police. Because if they actually go ahead and investigate the case and find out you were a recipient of the text message you could potentially be charged.”

Later Wednesday night, she spoke to Island parents at a session held at the YMCA.

On Wednesday morning she told high schoolers: “One of the main things I want you to think about when you leave here today is the responsibility you have on Facebook to hit the send or share button, the record made on these things could follow you guys for life. These little tiny posts or comments that you might not think are a big deal could totally change your life down the line so I ask you to think about that before you send something or post a message.”