File photo of a free Syrian army fighter carries his weapon in a damaged building in eastern Syria. PHOTO: REUTERS/FILE
On sites including Twitter, blogging platform Tumblr and the question-and-answer site Ask.fm, the young men are providing an unusual insight into one aspect of the brutal war.
They describe what they miss about home and what they ate for lunch, and extol the virtues of fighting with groups that Western governments deem "terrorist" organisations.
Britain, France and Holland each estimate that hundreds of their citizens are in Syria, and fear they could later launch attacks at home.
Ifthekar Jaman, 23, is from the British city of Portsmouth.
He was an active user of social media before he left Britain, maintaining several Twitter accounts, posting short videos on Keek.com and answering questions at Ask.fm.
He openly discussed his desire to leave Britain for Syria, and on May 14 tweeted he had made "touch down" in Turkey, before crossing the border.
He identifies himself publicly as fighting with the Islamic State of Iraq and the Levant, a group that emerged from al Qaeda's Iraqi affiliate, but rejects descriptions of the group as extremist.
"A man leaves the comfort of his home and everything else behind so that he can help an oppressed people. Sounds heroic, until you add in 'Muslim man.' Then he's a terrorist/extremist," he tweeted in November.
Much of Jaman's online activity is devoted to documenting everyday life in the conflict.
"Puppies are actually so cute, there was like three of them walking near by me," he tweeted alongside a picture of him petting a dog in November.
"Just to make it even better, there are two kittens living with us, subhanallah (praise God), so cute," he added in another tweet.
On Ask.fm, he encourages interested followers to submit their questions about his life, his motivation and the possibility of joining the fight.
He tells women they can come to Syria to help the opposition, though not in combat, and he reports meeting fighters from France, America, Canada, Australia and Finland.
"I knew only a tiny bit of Arabic," he assured one questioner worried about not speaking the language.
"There are many like you and you will fit in."
Other fighters offer similar assurances.
"For those worried about leaving the snacks behind," one fighter by the name of Abu Layth tweeted with a picture of bags full of junk food.
"Wallahi (I swear to God) those of you that are staying behind are missing out. Absolutely," he added.
Such public online activity is a relatively new development, according to Charles Lister, an analyst at IHS Janes' Terrorism and Insurgency Centre.
Earlier this year, there "were certainly Western foreign fighters in Syria but they were practically invisible", he told AFP.
"Literally over the last few months or so, that's started to be... increasingly visible."
"It's remarkably open. They don't seem to be keeping very much to themselves," he added.
"I think that's particularly unusual in comparison to other conflicts."
One fighter, writing under the name Chechclear, told questioners on Ask.fm he was not worried about the consequences of his public postings.
"I never came here with the intention of ever going back home," he wrote in a chat that included questions in English, Dutch and Turkish.
He compared his "jihad training" to that he said he received as a NATO soldier.
"NATO: Training, training, training. Jihad: Training, fighting, fighting and more fighting."
Lister said rebel groups were likely to have sanctioned the public Internet activity, possibly for its recruitment value.
"It's the personal level accounts of what it's like to be fighting on the ground, what the living conditions are like, what's recommended to bring with you... all of that is the really valuable stuff for someone considering going."
Many of the fighters sharing their experiences acknowledge that they hope to sway others to join them, like Twitter user Abu Fulan al Muhajir.
He began tweeting back in Denmark, sharing his dreams of going to Syria, and in August announced that he had arrived.
"The only reason I'm tweeting about my trip is to encourage other people to do the same. Everybody should do it," he wrote.
He tweets photos from battlefields, including a prison in Aleppo, described eating horsemeat from Russia for breakfast, and shared his disappointment at missing out on a battle.
"That feeling when you raise your hand and want to go on an operation and amir (leader) doesn't pick you. Naseeb (fate)," he tweeted in November.
https://twitter.com/Fulan2weet/status/397078117287985152
Chechclear, who has had his accounts on Twitter and the photo-sharing site Instagram suspended, recently began blogging on Tumblr, and made clear he understands the value of his online activity.
On November 26, he shared a photo of a knife, a handgun and a Samsung smartphone.
"Half of Jihad is media," he wrote underneath.
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Hope the western world wakes up and stops migration from these regions.