10-20% of apps developed by people on Hacker News probably use Heroku. It's not just a random service people use; if there is an outage, service operators will need to field end user inquiries, restore service, etc.
hn is probably not a replacement for a good offsite monitoring service, but for me, I was browsing "why does my friend's new app not work; is he updating it live?", switched tabs to hn, saw "heroku down", and all was clear.
I was going by other private surveys of specific subsets of the hn population. Number of companies, it's 10-20%. Lots of smaller companies or earlier stage; basically everyone who does Rails deployments (since that was originally Heroku's strength).
GAE is the one very few people are using or want to continue using, especially after the Google price hike.
I still prefer colo/managed hosting at scale, with EC2 for trials or surge capacity, myself.
hn is probably not a replacement for a good offsite monitoring service, but for me, I was browsing "why does my friend's new app not work; is he updating it live?", switched tabs to hn, saw "heroku down", and all was clear.