app/views/layouts/_welcome.erb
<% if user_signed_in? %>
<%= content_tag :span, current_user.email, class: 'email' %>
<%= link_to 'Log out', destroy_user_session_path, method: :delete %>
<% else %>
<%= link_to 'Log in', new_user_session_path %>
<%= link_to "Log in with Facebook", user_omniauth_authorize_path(:facebook) %>
<% end %>