Viral marketing versus word of mouth marketing
Here's something it's time to say outright.
If you're getting the impression from previous entries that you can't have any fun any more with all of those viral marketing tricks that people are so fond of, you'd be correct. Viral marketing is one of those poisoned well ideas, like affiliate marketing; too many spammers have hit too many people and systems for you to do it unless you are really good at doing it very lightly.
(You can get a certain amount of the innocent and the new to the net. But everyone who's been on the net for very long is overly familiar with getting barrages of invitations, notifications, or whatever from the latest website that one of their less wise friends has signed up with, and is not going to be impressed.)
So, what's the difference between viral marketing (bad) and word of mouth marketing (acceptable)? It's pretty simple:
Viral marketing is all of the things that you do and all of the features you build into your systems in order to make your website (or program or cause) spread. It is all of those automatic Facebook updates, all of those tweets, all of those clever plans to give people rewards if they turn over information on other people to you or get other people to sign up, all of those invites and reminders and notifications. In short, all of the things that you come up with to push your website in front of other people. When I phrase it that way, I hope it's clear why it's spam.
(Pushing things at uninterested and unwilling people is one of the moral definitions of spam. It's also one of the practical definitions of marketing, which is one of those eternal conflicts.)
Word of mouth marketing is your users telling other people about you (without the assistance of a 'click here to tell your friends' button). It is the enthusiastic tweet, the user-written Facebook status update, the 'you have to see this' email. There are two good things about word of mouth; first, it means that you have succeeded in creating genuinely enthusiastic users, and second, it's probably going to a reasonably receptive audience.
(And if they're not receptive, they're unlikely to blame you, they're going to blame their overly-enthusiastic friend.)
PS: I'm aware that this is pretty much a futile quest.