Bipul Lama believes Facebook is spying on him.

And he's got proof, sort of. Lama performed a test. For two days, all he talked about was Kit-Kats.

"The next day, all I saw on my Instagram and Facebook were Kit-Kat ads," Lama said.

After his Kit-Kat experiment, he successfully repeated it with chatter about Lysol. The 23-year-old musician is now more convinced than ever that Facebook is listening to his conversations through his phone's microphone.

"It listens to key words. If you say a word enough times, the algorithm catches those words and it sets off targeted ads," Lama theorized.

Lama is far from alone. The belief that Facebook is actively listening to people through their phones has become a full-on phenomenon. Facebook has, of course, denied it does this. That has done little to dampen the ongoing paranoia around the theory.

Because it is just a theory... right?

Facebook's covert ops?

Most people using Facebook understand that the social network is a giant data-collecting machine. Some users put Facebook's monstrous size and unknown goals out of mind, but others find it hard to dismiss. How much does Facebook know about them? What's Facebook using that information for? And how much of these concerns are paranoia, and how much is real?

"In many ways, of course Facebook is spying on you," said Brandie Nonnecke, research and development manager at the University of California Berkeley's Center for Information Technology Research in the Interest of Society. "It's not doing it for malicious reasons. It's trying to tailor content to you and advertisers."

Lama's experience seeing Facebook ads that seem to reflect his offline life is par for the course for many Facebook users.

"One time I was texting a friend about how I really wanted some clear-framed glasses, then I got a Warby Parker ad," said Olivia Reardon, a 25-year-old based in Boston. "Like how does Facebook know what I'm texting about? It reads my mind and my texts!"

Tweet may have been deleted

Tweet may have been deleted

Tweet may have been deleted

There's a reason Facebook gets a worse rap for this than, say, Google. With Google, it's just ads. With Facebook, it's personal. The ads and creepy suggested friends and prompts to celebrate holidays that may or may not apply all add up to something more unsettling.

Barring leaving Facebook and Instagram—a choice that would be untenable or extreme for many—even privacy-aware users don't feel like there's much they can do about it.

"I've tried to look in privacy settings before but it's not very clear," Reardon said. "It's not like you can check an option that says 'stop creeping my private conversations.'"

A common feeling

Facebook's just-creepy-enough ads and suggestions are perfectly built to cater to the psychological tendencies we already have. It's part of the reason why the idea of an all-knowing social network has such staying power even when aspects of Facebook's reach—like the microphone theory Lama favors—have been debunked.

"In their thinking about these kinds of online privacy issues, people rely on heuristics—mental shortcuts or little rules of thumb," said Shyam Sundar, a professor at Penn State who studies the social and psychological effects of online communication.

"Because we are bombarded with a lot of information, we tend not to spend time thinking about why our personal information ended up there. First we tend to be pissed off at the service or get annoyed or embarrassed," Sundar said. "Then we apply some sort of shortcut like, 'If I go to this site, it's going to end up embarrassing me."

Basically, we rationalize things. And sometimes that rationalization is jokingly saying "Facebook is spying on me" instead of investigating the details of the data we're giving the social network.

People also only notice things they're already thinking about. All the ads on Facebook that don't eerily align with your life go unnoticed, but the one that's pinpointed to your conversation last week jumps out.

When people see a creepy ad, they tend to think of the thing they did in real life that Facebook couldn't possibly know about. Sometimes, they forget about their online behaviors that, even if not as directly, might indicate similar interests to advertisers.

"If I'm discussing with my husband trying to have a child and I see things on Facebook about children, it's likely I'm searching different things that also indicate I'm trying to conceive," Nonnecke said. "The way that we interact with others in the day-to-day is very similar to what we're doing online."

The catch

All of this wouldn't be that big an issue if we knew what Facebook was doing. But it's still murky. Even with all the research explaining how humans interpret these kinds of coincidences, anything could be going on behind closed doors in Menlo Park.

One reason people are so sensitive to potential intrusion from Facebook is that they have the sense that something is off. Since Facebook isn't exactly forthcoming with details, users assume the worst. And they're not necessarily wrong.

"Companies are studying [users] closely," said Ryan Calo, an associate professor of law at the University of Washington who studies law and emerging technology. "There's no transparency in that process, so they assume other correlations—which are in fact coincidence—are part of that."

Until Facebook gives us the details on how exactly it's using 2 billion people's data, the paranoia is here to stay.

"What's the saying?" Calo said. "'Just because I'm paranoid doesn't mean no one's out to get me.'"