It's not about the amount of users but how the developers developed and configured their softwares. If it is about the user base then all of the major servers and mainframes like Google, Facebook, IBM, Amazon, and etc would have been compromised because they all use Linux.
If a user only do basic computing then he should be using Linux instead
Facebook has been hacked a few times, as I understand it. Amazons been hacked with some massive password and credit card detail leaks. Not sure I'd trust amazons security too much given this.
Google has their 'Google Vulnerability Reward Program' where they pay anyone that hacks them. And I'm talking decent pay here - find an exploit and get thousands of dollars. Googles been hacked in the past, or at least there are credible claims, but I think they take patching vulnerabilities incredibly seriously.
Amazon also has its own bug bounty program, even though they have been majorly hacked at least twice.
So its really not a case of Linux can't be hacked, its more that people who run Linux servers are beholden to constantly plug exploits, much like desktop OS makers have to constantly fix exploits.
The problem with desktops, is those people don't update their software, fix the holes - or if the exploit is discovered by the hacker before the security people. Its really more like an arms race. There is no OS in the world which is unhackable.
Most of the hacking is actually done by governments attacking other governments, or corporations or governments spying on citizens or other governments/corporations. Hacking is big money - if your good at it, you'd be a fool to not work for somebody.
Using a desktop Linux platform, is for security purposes like using a sailfish, windows 10 mobile, or bb10 phone - there's not many people using it, so that's going to make you pretty safe on its own.
Linux does have a pretty tight permissions system. that is slightly better than windows though. But it also doesn't sandbox applications, which windows 10 uwp apps do maiing it vulnerable to malware exploits, should anyone be motivated to write one.
Android of course users Linux's permission system, but its still possible to get in through the user, via malware or other related exploits. It's a little more complicated, as one has to essentially exploit one's way through the layers of permission levels to get root access, but its not impossible. as many have proven.
Would I say Linux is more secure? Yeah it probably is, at a basic level. But who's plugging the exploits when it is vulnerable? If it were mass adopted, someone would need to seriously, commercially manage that end. I wouldn't trust a free open sourced distribution, if engaged at a massive consumer scale to be secure at all - because every exploit that exists would have to be patched by a centralised software distribution agency, and with some rapid speed.
If Linux had a centralised application store, like windows or osx, running sandboxed applications that have been through some level of basic scrutiny, AND it received centralised security updates, via automatic update - it would undoubtably be the most secure OS there is, very very hard to hack. It already has some strong advantages in terms of security, no doubt. But invulnerable - no, it ain't.
Plus Linux in a way is a sort of legacy OS. It's going to struggle to keep up in the age of machine learning, and VR/AR, multiple devices hybridization and so on. There is SO many coding hours involved in these sort of projects, and because Linux is open source, it tends to lag behind.
Something like Linux mint for example, can't run on a smart phone, can't run in mixed reality. Linux has no smarthome hub, no voice assistant, no machine learning apis. It's hard not to imagine Linux being left behind in what essentially is another arms race- the race between google, Samsung, amazon, sony apple etc for the future.
Certainly if it could become competitive in these future areas - mixed reality, machine learning, voice recognition, hybridization/form conversion, varying input and output methods (voice, stylus, touch, gesture, hologram projection, little screen, no screen), as windows 10 is trying to do. If it could even keep up, it would offer a competitive advantage over the others paid or advertising funded offerings.
But jesus those unpaid open source devs would have to literally work their behinds off for a great many years. I can't see that even being possible without commercialisation. Even then, there's the software gap. You'd have to market it well enough, or have enough of a competitive offering vs the others, that users would switch.
Its not only possible for billions of people to be wrong at once, its probable that the on average opinion is usually wrong. 
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