Sometimes we need to make our project more 'social', so we add 'share' ability, log in with (Facebook, Twitter, Instagram, etc), duplicating posts in social networks. There is a bit set of gems and articles in this field.
Habr post: devise + Omniauth + Facebook + VK (RUS), Devise wiki
Although you can add 'log in with ...' links to devise forms if add this code to:
<!-- app/views/devise/shared/_links.html.erb -->
<% if devise_mapping.omniauthable? %>
<% resource_class.omniauth_providers.each do |provider| %>
<%= link_to "Sign in with #{OmniAuth::Utils.camelize(provider)}", omniauth_authorize_path(resource_name, provider) %><br />
<% end %>
Flexible gem with cute button images. Supports: Facebook, Twitter, Douban, Google+, Weibo, QZone, Google Bookmark, Delicious, Tumblr, Pinterest, Email, LinkedIn, WeChat (Weixin), Vkontakte, Odnoklassniki, Xing, Reddit, Hacker News, Telegram.
- RC::Dropbox (via OAuth 1.0a)
- RC::Facebook (via OAuth 2, most completed)
- RC::Github (via OAuth 2)
- RC::Instagram (via OAuth 2)
- RC::Linkedin (via OAuth 1.0a)
- RC::StackExchange (via OAuth 2)
- RC::Twitter (via OAuth 1.0a)