Sorry for being a bit subjective but how can we persuade other people we know to use Ubuntu...
I've brought a number of people from Windows to Ubuntu. There are several points that usually end up being the strongest sells:
They want to have me as a technical resource when they have computer problems. I tell them I will only support them if they run Ubuntu, since I don't really use Windows myself anymore myself.
They are suffering with spyware and viruses on Windows and want to escape that, but don't want to shell out for a new computer. They like the idea that Ubuntu is immune to all this tends to resonate with them.
I try to always have a non-technical person like my wife with me, as they can speak more peer-to-peer and reassure them that you don't need to be a computer expert or anything. They also tend to be able to explain things in a way that newbies can understand, when I'd be giving Too Much Information.
I usually leave all the stuff about the free software ethos as secondary, just frosting for them. Some people groove to that, but for most people it's not a selling point; if it were, they'd probably have tried out Ubuntu already.
I also don't talk about the free cost of Ubuntu. That can make them think, "Hmm, must not be any good". Instead I'll say, "You pay for a support contract - that's how they make money, and they're actually pretty good. But that's optional, if you don't think you'll need it, you can use it without paying anything, on as many computers as you want."
My advice is restricted, but persistent advocacy: tell your friends (or Facebook "friends") about all the good stuff, but don't be too pushy. Although Ubuntu has many virtues, so does MacOS X, and even Windows has its advantages - not to mention the other Linux distros. Don't forget that when praising your system of choice.
Actions speak louder than words. Show how much can be accomplished in Ubuntu, how elegantly it works. In my experience, showing the obvious advantages really makes an impression: the system is fast and reliable because it doesn't need a virus scanner; it comes with a full-featured word processor preinstalled; it has beautiful desktop-effects only a mouse click away; for the most part it works instantly, without any driver installation.
Two more things are really appealing and ought to be highlighted: the liberty of free software, with all its ethical and legal advantages, and the community of bright and helpful people - ubuntuforums.org, wikis, and now ubuntu.stackexchange.com. Starting with these, you won't persuade everyone, but you may convince some, and those may be the ones who contribute back to Ubuntu in the future.
you can't: it's a matter of honor. if one believes that he is on top of the time with windows, you can't change their mind. nobody is pride to use a os for free even if he has no money for windows, they will rather use an old cracked XP... So while this also works they have at least the experience to work with the os of the biggest software producer.
that's just my 5 cents.
How I Promote Ubuntu :
Using It Myself: People who walk past me are curious and ask "What is that ? Windows ? " . Then I tell them it is "Ubuntu".
Bringing ISO's and CDs on-the-go: While some people do know what Ubuntu is ,they may have in mind that "Nah ,Windows is better " . Offering them a CD or an ISO can let them try out Ubuntu and they may even install it.
I'm on a project (by myself) to offer free Ubuntu CDs to all the people who need them. Although CDs are redistributions , it is being shipped free of ANY charges to people (if they are in Malaysia).
Anyways , I'm sure there are a lot more ways we can promote Ubuntu. You can check out this website if you want some Promotional Materials. The Spread Ubuntu website contains a variety materials such as brochures , posters , packaging and more.