Rails 4 form: checkbox not saving value to database

I have a Rails 4 app with the following Post model:

create_table "posts", force: :cascade do |t|
    t.integer  "calendar_id"
    t.date     "date"
    t.time     "time"
    t.string   "subject"
    t.string   "format"
    t.text     "copy"
    t.datetime "created_at",         null: false
    t.datetime "updated_at",         null: false
    t.string   "image_file_name"
    t.string   "image_content_type"
    t.integer  "image_file_size"
    t.datetime "image_updated_at"
    t.string   "short_copy"
    t.integer  "score"
    t.boolean  "facebook"
    t.boolean  "twitter"
    t.boolean  "instagram"
    t.boolean  "pinterest"
    t.boolean  "google"
    t.boolean  "linkedin"
    t.boolean  "tumblr"
    t.boolean  "snapchat"
    t.string   "approval"
  end

Users must be allowed to decide whether post.facebook is true or false, which is the reason why :facebook is a boolean.

Then, I created the following form to allow users to actually make post.facebook true or false:

<%= form_for [@calendar, @calendar.posts.build], html: { multipart: true } do |f| %>
  <% if @post.errors.any? %>
    <div id="error_explanation">
      <h2><%= pluralize(@post.errors.count, "error") %> prohibited this post from being saved:</h2>

      <ul>
      <% @post.errors.full_messages.each do |message| %>
        <li><%= message %></li>
      <% end %>
      </ul>
    </div>
  <% end %>

  <hr>

  <div class="field">
    <h3>Post details</h3>
    <p>
      <%= f.label :date %>
      <%= f.date_select :date %>
    </p>
    <p>
      <%= f.label :time %>
      <%= f.time_select :time %>
    </p>
    <p>
      <%= f.label :format %>
      <%= f.select :format, ['Simple Status', 'Image', 'Link', 'Video'] %>
    </p>
    <hr>
    <h3>Post content</h3>
    <p>
      <%= f.label :subject %>
      <%= f.text_field :subject %>
    </p>
    <p>
      <%= f.label :copy %>
      <%= f.text_area :copy %>
    </p>
    <p>
      <%= f.label :short_copy, "Short copy (Twitter only)" %>
      <%= f.text_area :short_copy %>
    </p>
    <hr>
    <h3>Social channels</h3>
    <p>
      <%= label_tag(:facebook, "Facebook" %>
      <%= check_box_tag(:facebook) %>
    </p>
    <p>
      <%= label_tag(:twitter, "Twitter") %>
      <%= check_box_tag(:twitter) %>
    </p>
    <p>
      <%= label_tag(:instagram, "Instagram") %>
      <%= check_box_tag(:instagram) %>
    </p>
    <p>
      <%= label_tag(:pinterest, "Pinterest") %>
      <%= check_box_tag(:pinterest) %>
    </p>
    <p>
      <%= label_tag(:google, "Google+") %>
      <%= check_box_tag(:google) %>
    </p>
    <p>
      <%= label_tag(:linkedin, "LinkedIn") %>
      <%= check_box_tag(:linkedin) %>
    </p>
    <p>
      <%= label_tag(:tumblr, "Tumblr") %>
      <%= check_box_tag(:tumblr) %>
    </p>
    <p>
      <%= label_tag(:snapchat, "Snapchat") %>
      <%= check_box_tag(:snapchat) %>
    </p>
    <hr>
    <h3>Image</h3>
    <p>
      <%= f.label :image %><br>
      <%= f.file_field :image %>
    </p>
  </div>
  <hr>
  <div class="actions">
    <%= f.submit %>
  </div>
<% end %>

And I did sanitize all parameters in my posts_controller.rb:

def post_params
  params.require(:post).permit(:date, :time, :subject, :format, :copy, :image, :short_copy, :score, :facebook, :twitter, :instagram, :pinterest, :google, :linkedin, :tumblr, :snapchat, :approval)
end

The problem is, whenever I try to create a new post or edit an existing post, and check the :facebook checkbox, then save the post, the value is still set to nil.

How can I get the following behavior:

• Default value of post.facebook => false
• Default state of the :facebook checkbox => unchecked
• Value of post.facebook when the :facebook checkbox is checked => true
• State of the :facebook checkbox when the :facebook checkbox is checked => checked

I found the following Stack Overflow questions but could not figure out a solution to my issue from there:

• Ruby on Rails - Checkbox not saving to database?
• Rails Checkbox not working - no error when submitting form
• Get checkbox to render as checked if boolean value is false

This seems like a basic requirement / problem, and I believe I followed the Rails documentation, but I can't figure it out.

Any idea of what I am missing / doing wrong?