Although many people equate the internet with the Web, and email, there are actually lots of other ways to communicate over the Net. Some let you talk to other people if they are online at the same time - they are called 'synchronous'. Others let you leave messages which people can pick up later - they are 'asynchronous'. Many websites offer instant messaging services which let you talk to other people online.
Email is essentially private, like written post.
But there are also lots of public spaces online where you can leave messages.