The Pachube service was acquired several years ago by a company called LogMeIn.
They changed the name of the service to COSM. A short while later, they renamed it Xively.
The Xively service used to exist in two forms: Xively (Commercial) and Xively Personal.
In January 2018 they shutdown Xively Personal.
You should check out the blog of a fellow Netduino user/developer who goes by devMobile. He evaluated many alternatives to Xively and has published some findings.
He has posted code for interfacing with a different IoT service called Adafruit IO. His code is very clean & elegant.
I have run his code successfully on a Netduino 3 WiFi (but have issues with a Netduino 2 Plus).
I have used both Xively Personal and Adafruit IO. Adafruit IO is much more straightforward to use.
Adafruit IO can be found here: Adafruit IO
Here is a link to the particular entry on devMobile's blog...
devMobile's Blog - Xively Personal is being retired