The photo of the shirtless teenage boy appeared on Nicole Lovell’s Instagram account around the beginning of December. Next to it, a message was posted under the 13-year-old’s username: “I love this baby he’s so hot. HES MINE.”

On Facebook, Nicole posted a selfie with her lips pursed and hair swept to one side. A man from India posted a reply that read, “I love you.”

Less than a month later, prosecutors said, Nicole was killed by 18-year-old Virginia Tech student David Eisenhauer, whom she met online. Like many teens, the small ups and downs of young adulthood played out across Nicole’s Facebook, Kik and Instagram pages, but her online life also highlighted some of the dangers of social media for teens — a world many parents know little about.

Friends said, and her social media accounts show, that the teen from Blacksburg, Va., corresponded with more than one man online and she hung out in an explicit teen dating forum. Now, the high-profile case has some parents and young people alike reconsidering how they use social media.

Marissa Cook, 17, is a high school junior whose younger sister Kari attended middle school with Nicole. She said Nicole’s death has frightened some girls into getting offline altogether, wiping Snapchat and Kik from their phones.

“I think it’s really opened some eyes,” Cook said. Both Cook and her sister said they were on Kik but deleted it after getting lewd photos from strangers.

Tammy Weeks, Nicole’s mother, said her daughter was bullied in school and often cried, asking to stay home. She seemed to seek solace online. Nicole posted typical photos of herself posing with stuffed pandas, her siblings and hugging her father, but she also poured her heart out.

In many social-media posts, Nicole projected a girl who was heartbroken, suicidal and deeply vulnerable. She posted a self-portrait to Instagram eight weeks ago, her face covered by her hand save one blue eye.

“Forget about me please I don’t need no help or friends they will just stab me in the back anyway,” she wrote. With another forlorn-looking self-portrait, she posted, “I want him back so bad,” though it’s unclear whom she is referring to.

And 12 weeks ago, she posted to Instagram an image of the text “I’m going to kill myself” and crying emojis.

Nicole seemed unafraid to put herself out there; as one friend put it, she “wanted to be wanted a lot.”

In January, Nicole posted a short message to a Facebook group called “Teen Dating and Flirting.” It featured a close-up selfie and the message: “Cute or nah.”

The message received hundreds of replies. Many were ugly, and the messages were likely little different from the bullying she encountered at school.

“You’re very round,” one person wrote.

“Teen Dating and Flirting” had more than 18,000 members before Facebook shut it down after Nicole’s death and complaints from child advocates. Members had openly solicited sex and asked for nude pictures, and the page was littered with spam for “Sugar Daddy” sites.

In a post on Instagram, Nicole made a plea to a teen who appeared older than she: “You are the first guy I have ever cared about please just come back to me.”

Kari Cook, 13, has said that several weeks ago she alerted a school resource officer at Blacksburg Middle School after Nicole posted on Facebook a photo of an older teen she seemed to be dating. Cook said the man did not appear to be Eisenhauer. Blacksburg police said they never received such a complaint. A second friend said he also became alarmed at Nicole’s interactions with men on social media.

“In heaven don’t hang out with random guys,” a friend wrote in a memorial to Nicole on Instagram. “I gave you this advice when you were standing and sitting right in front of me but you never listened.”

Nicole disappeared from her home on Jan. 27. A neighbor told the Associated Press that earlier that day Nicole told the neighbor’s daughters that she had a boyfriend named “David” and showed messages between the two on the instant-messaging app Kik. Nicole’s body was discovered in a remote stretch along the Virginia-North Carolina border last weekend.

A prosecutor said at a bond hearing Thursday that Eisenhauer, of Columbia, Md., and another Virginia Tech freshman, Natalie Keepers, 19, of Laurel, Md., conspired to kill Nicole over the month of January, buying a shovel and picking a remote spot to slit her throat. The prosecutor did not offer a motive for the killing, but two law enforcement officials have told The Post that Eisenhauer had sexual contact with Nicole and that he used their relationship to lure her out of her home.

Nicole’s death has heightened scrutiny around certain social-media apps, where adults with inappropriate intentions can have ready access to children online. Terri Lovell, Nicole’s stepmother, created a GoFundMe page to raise money for funeral expenses and for “promoting online safety for our children.”

One 14-year-old girl at Blacksburg Middle School did not know Nicole well but said she and her friends have been leery of social media ever since a stranger messaged her friend on Instagram asking for nude pictures. She said that some middle-schoolers invent online personas making themselves appear older than they really are.

“They want to date older boys, I guess, so they pretend to be older,” the girl said.

Tiffany Moeltner’s two daughters attended Blacksburg Middle School with Nicole. For many reasons, she restricts their access to social media, even reviewing their followers on Instagram and limiting their phone time to 30 minutes on weeknights. One reason, though, is the prospect that strangers could reach out to her daughters on social media.

“The social dangers of people phishing and connecting with the kids is bad enough,” Moeltner said. “It’s really scary.”