Long as we're mixing facts with biases and opinions, why not?
Linux is, for the most part, invincible as of right now. Not everyone at your local Starbucks uses Linux. Some exploits can be done through browser (website scripts) but I don't much in this domain. I don't Internet much.
How many Windows viruses (virii?) are out there now? Linux/UNIX is vulnerable to forks (and decompressions, and tarbombs) and some commands. That's why Ubuntu (cannot speak for other distros) has a huge software database (really kinda like AppStore) and everything in there is verified by Ubuntu (Canonical folks). You can add untrusted PPAs at your own risk, but the most danger really comes to someone telling you to get a tarball and you actually decompress the thing without verifying it (or the source). Same thing as commands. You simply do not download (or copy-paste) a script and run it blindly without going through some basic bash scripting reads. Correct,
no system is safe (copyright EA Games). However, do you project that somewhere in the future, Linux distros replace Windows? I can't speak of Mac OS but their hardware are ridiculously priced (personal opinion).
Let's get back to security.
Here's how to install a virus on Linux. Software (packages) enter a Ubuntu system (again, cannot speak for other distros although I believe they're all pretty similar) by three ways: Central (software center, trusted), PPAs and pre-made packages (.deb, .rpm. YUM etc, untrusted as in manually added by the user) and source files (tarballs, need to be literally "made" by the user). Through Central there is little, if at all, chance of getting compromised (there is a bit of risk of man-in-the-middle). Pre-made packages are somewhat like your average Setup.exe files, there are downloaded either by you or by adding a PPA. With Windows, a new user with little security knowledge can surf around and click on a bunch of stuff, some good, some useless yet benign, some viruses/trojans. It is the same thing with Linux, only distro-specific pre-made packages are much harder to come by (as you have implied, companies see Linux as unprofitable). So this is a risk, yes, but if you are looking for something specific, there are support forums (and most exist in Central anyway). Several Linux magazines can introduce some pretty good stuff (Eagle Mode, for one) that doesn't exist in Central, not even a PPA. This relies on you and how you think. Do everything that a magazine tells you? I got to know and installed Eagle Mode and XBMC through reading magazines (the latter is already in Central).
The most risk is within tarballs. I do not have much experience with tarballs, but whatever tarball I came across, there is the same (or equivalent) in package form, or better, in Central. If one wants to make a tarball (literally
cd directory and
make), one should read the files first. Again this demands scripting knowledge, so at least I don't blindly make tarballs.
You do need to be careful for rootkits (although rare). Ubuntu provides
rkhunter and
chkrootkit. Run them periodically (I have to say, mostly overkill).
Now the virus/malware part is mostly covered, what's left is the Internet part. This I cannot say much. Anyone with a package sniffer can get your transmissions. Again, no system is safe. Windows relies on AV and firewalls; Ubuntu relies on
this. There are complicated procedures for setting up port rules and whatnot, a basic line is to enable ufw. However the kernel shipped with Ubuntu is already firewalled (all ports are closed). If you want to go through a lot to set up rulesets for your ports, that makes me ask: What makes you think that you will be targeted in the first place? Do not fear the average Windows user; fear instead the non-average Linux user. But this loops back to Linux is invincible: when do you think that Linux is going to become mainstream? What makes someone develop skills to attack something that so few people uses? There are 500,000 people in Quebec City: to date I count 5 Linux users (three Ubuntu, one METI one Mandriva) including myself, which I'd say would be 100 Linux users in all of Quebec City (and more in Montreal, of course). Simply go to your local WorstBuy and look at what their computers' OS is.
Then there's always physical attacks. Encrypt your BIOS, encrypt your home folder, backup your data, wrap some chains around your computer, etc etc. But does this apply only to Linux?
Is Linux more secure than Windows right now? Yes. Will Linux remain so in the future? Quite so, until the day 85% of the computer users run Linux. Mac viruses exist; Macs are getting mainstream (or non-conformist like everyone else). Let't wail til the day where there is a Linux section at your WorstBuy, and Linux Stores on 5th Avenue.
EDIT: To avoid getting socially engineered, just don't register Failbook.