is there a way to share an event post to facebook or mailchimp?
In CiviCRM you can create events and manage (registered) participants at an event.
CiviCRM also provides an online registration form for events.

You can use the link to the public information page and the online registration form in a mail send from mailchimp or in a message on facebook. As far as I know there is no automatic way of doing this.

If you want more information also take a look at the user guide of CiviCRM: https://docs.civicrm.org/user/en/latest/events/what-is-civievent/