Simple Messaging Service
In my research into building an Interactive forum and the comparison of how Facebook and Wordpress handle notifications, the ability to generate a message from the forum seemed to be a requirement.
The thing to be aware of is that text messages cost money to send. This can be absorbed into the operation of the website. For example, when your bank or the government send you a confirmation/security code they are paying for the message. If you have an account with them then this pays for the message. In the case of the Government then it is your taxes that pay for it!
Push Technology
This is the technology that allows messages to be "pushed" to Internet users if an event occurs and the vendor of the platform wants its user to know that it has occured. Most frequently this is an update or a comment on a Social Media post/profile (that is being "followed") or it could even be anm email.
WhatsApp is a cross-platform messaging app, even though it is brought to you by Facebook. Facebook probably realised that not everyone uses their platform and therefore they needed a program that allowed non-users to communicate. This is me!
I have good and bad experiences with WhatsApp. The good is that I was able to communicate with my partner both by text and video when I had a section of my colon removed. The bad is that I mistakenly talked to another WhatsApp user, which only confirmed my distain for them!