First of all, include OmniAuth to your rails app by this railscast or this.

Then add to Gemfile this gem:

gem ‘fb_graph’, ‘1.5.4’

After, you should turn on permission in your apps:

for Facebook:

for Twitter:

When your login from OmniAuth, social app ask your permission on update wall in Facebook and post tweet in Twitter.

Paste to you authentication model (my model named Person):

Publish message:

@person.publish(“Hello, my Facebook or Twitter friends!”)