Rails radio buttons checked with params

I'm sorting through the User model on the User Index page. Right now, all I have is two options that can be accessed through radio buttons. I may be doing this the wrong way, it works for now, but if you have a better way then please shed some light. The main question I have is why are the radio buttons not checked when they are selected?

User's Index Controller

if params[:sort] == "popular"
  @users = User.most_popular_users
elsif params[:sort] == "recent"
  @users = User.most_recent_users
end

User.rb

scope :most_popular_users, -> do 
  results = select {|user| user.followers }
  results.sort! {|t1, t2| t2.followers.count <=> t1.followers.count}
end

scope :most_recent_users, -> do 
  results = User.all
  results.sort! {|t1, t2| t2.created_at <=> t1.created_at}
end

Again, I'm not sure how correct that is in terms of "best practices", so if it's wrong I would love to change it as well. Back to the question...

My view looks like this:

<%= form_tag find_users_path, :method => 'get' do %>
  <%= radio_button_tag(:sort, "popular") %>
  <%= label_tag(:sort_popular, "Popular") %>
  <%= radio_button_tag(:sort, "recent") %>
  <%= label_tag(:sort_recent, "Recent") %>

  <%= submit_tag "Submit" %>
<% end %>

Right now, when you submit the form, it sorts the Users properly, but the selected radio is not selected on the respective page. So when you submit the form the find the popular users, the url looks like:

/people?sort=popular

but the radio button for popular is not checked.