It's been 15 years since Mark Zuckerberg founded Facebook, the most influential social network in the world.
At that time, Zuckeberg was 19 and lived in the student residence of Harvard University. It was there where he launched the first version of Facebook, "a simple web to get in touch with what really matters: people," he explains in his most recent post.
Aaron Greenspan met him during that university time. They studied together at Harvard.
Become a technology entrepreneur - he is the president and CEO of Think Computer - Greenspan is now a vocal critic of Facebook. His latest accusation came to light at the end of January: a 75-page report in which he claims that half of the social network's accounts are false.
Facebook flatly denies it: "It is unequivocally false and irresponsible," the network said in a statement.
But Greenspan insists. And he says that Mark, his friend from college, betrayed him.
From friends to rivals
"Mark and I became friends when we studied at Harvard," Greenspan tells BBC World.
"I was one of the few people on campus who was interested in technology and business, which was quite strange at Harvard at the time."
"We met at a dinner on January 8, 2004," he recalls. "He seemed a little strange to me, but we got along well, we stayed in touch for a while, then he told me that he considered me his friend."
But the reason that separated him from Zuckerberg was the same that united him: to create a social network.
"What irony, Mark, who has defined friendship for virtually an entire generation, does not even know what that word means," says Greenspan.
"I created my own Facebook, 'The Facebook' in 2003. Mark copied me several things and put them on his Facebook [which was also called 'The Facebook'] for a short period of time, from February 2004 to May or June 2005, there were two 'facebooks' managed by Harvard students, "says the computer engineer.
The Greenspan was a system originally called houseSYSTEM to get in touch with other university students and turn university social life into a digital one.
"The idea of creating an integrated student portal that had a biography, a calendar of events, photo albums, groups ... was mine," says Greenspan, who says he sent an email to Harvard students announcing his four platform months before Zuckerberg launched his.
He considers that Zuckerberg betrayed him and "used him for his benefit".
"I underestimated his ability to act so interested, I still thought then that Mark was my friend, because he told me so, I was upset with him on a personal level, but I could not really understand what he did or why" declares
"I did not realize I was a totally interested person until much later, and I should have done it before."
"Many, many years have passed since then, I no longer have any hope that Mark will continue to be my friend, and now I see Mark in the same way many people see him: as a person I do not know well, as someone who has a huge impact on the lives of many people, and I would say that that impact is now, to a large extent, negative. "
Greenspan reached a confidential agreement with Zuckerberg in 2009, when the CEO of the social network tried to register the word "facebook" as a brand.
But the dispute did not end there.
The false accounts
In the report he published on January 24, Greenspan tells that Facebook is "the biggest scam in history" and that half of its users are fake.
Greenspan says that the social network never wanted to recognize the extent of the problem of false accounts that came to light with the Cambridge Analytica scandal and the 2016 US elections. He also says that, in theory, that problem has no solution.
"Facebook is so big and the team, in proportion, is so small, they do not know what's happening," he tells BBC Mundo.
"They created a structure in which, from a computer perspective, it is not possible to do what they say can be done."
"They say they can distinguish between a real and a fake account, in some cases it's possible, if, for example, it's very obvious, but if someone says their name is José Pérez, how do you really know who he is?"
"The Turing Test, which was developed by Alan Turing, states that, basically, for a computer to be intelligent it has to be able to distinguish between a human and a machine," argues Greenspan.
"And the issue is complicated because it would be necessary to determine if that entity is a human and, in addition, to say what human it is, I do not think Facebook can do that, and I do not think I'll ever be able to do it."
Greenspan thinks that Zuckerberg was lying when he declared before the United States Congress that he will solve that question thanks to artificial intelligence.
"It's not true ... and he knows it's not true," he laughs.
And he says that he warned Zuckerberg much earlier about this, in 2005.
"Mark knew about that problem since then, I told him in writing and we had a conversation because a friend of a friend had a problem related to that issue, Mark denied it and said we were exaggerating things."
"He knew it and he knew exactly what could happen, and that fact is important, but he never recognized it."
Greenspan believes that Zuckerberg should not only resign, but also be prosecuted for the damage his social network is causing.
"A lot of people have underestimated how far Zuckerberg could go to get more power, I also did it, it was my mistake, but I think he was always like that, he has no empathy for people, he is able to break laws, break promises and ruin relationships to move forward, "he says.
Defenders of Zuckerberg have been demonstrating on Quora and on other websites claiming that Greenspan attacks Zuckerberg because he has resentment towards him.
Others call him an opportunist for attacking Facebook's founder at a particularly sensitive moment.
"The context of my relationship with Mark is important and I understand why they question it," answers Greenspan.
"Many say that I am bitter, that I am dying of envy, that I am jealous ... and many other things, but I have been saying the same thing for 15 years, what happens is that before they ignored me and thought I was crazy. it's like that. "
"Also, this goes beyond my personal history with Zuckerberg and beyond Facebook, it's a symptom of what happens when people are not held accountable for things." If Mark had been held accountable in college, we would not have gotten here. "