Instead of linkify, to get pretty twitter hashtags and mentions try the twitter-text gem.
Add to your Gemfile
gem 'twitter-text'
Then add to your application_helper.rb
module ApplicationHelper
include Twitter::Autolink
# Whatever else you have going on in your helper
end
Then in your view
<% @tweet_news.each do |tweet| %>
<li><em><%= l tweet.created_at %></em>
<p><%= auto_link(tweet.text) %></p>
<li>
<% end %>