On Dec 17 2015, one of our competitors, Pingdom sent an email like below to its free users.
We are sending you this email to notify you about changes to your free Pingdom account. In an effort to deliver the best products and support for our customers that subscribe to our paid plans, we will no longer provide your existing free plan after January 28, 2016. ……
If you have a free account with Pingdom, your account is now significantly reduced in functionalities (5-min instead of 1-min monitoring interval, no public report etc).
You also cannot get a new free account from Pingdom. Their free plan sign up page now shows a free trial instead.