The feature is also integrated into the News Feed: after a user has liked or commented on a representative’s FB post, a new post may appear prompting to “Contact Your Representative.” Facebook will also include local election reminders at the top of News Feed.
To access Town Hall from a desktop, look under the "Explore" section of your News Feed. To access from a mobile phone, look in the menu section of the Facebook app.
(image from Mark Zuckerberg's FB post)