I agree w/r/t to the facebook apps - I don't think they have any lasting value.

And I think any product that completely revolutionizes a domain doesn't need to "go viral" to succeed - ultimately the excellence or game changing experience will cause these apps to succeed, Amazon, and the iPod are two great examples. Google was a success because they created a game changing search product - it was (and still is) vastly better then anything else out there.

I think however that while everyone tries to create a game changing product, the vast majority of products are really incremental improvements and in these cases going viral helps. Twitter being a great case in point - really just an iteration somewhere between IM/Email/Blogging - but went viral enough to be important, over some better platforms such as Jaiku.

That's an important distinction to make re: virality. There're two kinds: the sort that forces you to spread it to gain functionality, and the sort that spreads because people like talking about it. It's easy to force the one, but very, very difficult to fake the other.