I can think of many services that have rapidly outgrown their ability to serve pages to the users in a seamless fashion (people regularly complain about reddit, though I don't recall encountering it myself (but then I'm only on reddit rarely), and people complain about twitter (were they the guys with the fail whale?)). And various blogging software also spits its lunch (wordpress?)

Then you have MySQL which regularly died under load. I think it's better now than it used to be.

I guess the counter-counter example is that even if your technology foundations are crappy you can still become popular if the service is compelling enough to override minor quibbles like 'stability' and 'scalability' (don't tell the architects I said that :D )