[chinathrow]

We've seen the same with Pingdom. ISPs are (mostly) multi-homed and Pingdom might use just one (affected) route to the ISP in question and then fail badly.

If Pingdom can't get to your site, it's highly likely your users can't either.

[teddyh]

That was not the case with us; Pingdom would report short outages, like a few seconds here, a couple of minutes there, and only a handful of occurrences for the whole report duration (IIRC).