How to post the node link in social networking if node is published?
For Facebook and Twitter you should be able to get this to work using the Rules module, though for Google+ this sounds like a business opportunity (read: no known solution as of today). Read on for more details about each of them ...

Facebook

The Facebook Rules module can be used to post to user's wall, via the Rules module. Some more details about Facebook Rules (from its project page):


  Facebook rules module extends FBOauth permissions and integrates with Rules module.
  
  Once the user's Drupal Account is connected to his/her FaceBook Account, FaceBook Rules can be used to set up actions based on particular events or triggers.
  
  It provides following rules:
  
  
  Events:
  
  
  User logged in using Facebook connect.
  User connected existing own account to Facebook.
  User registered using Facebook connect.
  User registered using Facebook connect, requires admin approval.
  
  Condition: User's Drupal account is connected to Facebook account.
  Action: Post to user's wall.
  


Twitter

After you enable the Twitter module, and its sub-module Twitter Actions, you can use the Rules module to create an action that will post a tweet to a specific Twitter account.

Have a look at Post Automated Tweets Using Rules and Twitter Actions for more details about it.

Google+

For Google+ I'm not aware of any working solution. Yes there is the Google+ module (which only has a dev version for D7 ...). And yes Google+ does have an API, but this module doesn't implement it.

Therefor I think the best you can find (to start from) is what is described in How to post in google plus wall?. Starting from there, you may get that to work also if you'd write your own (custom) Rules (Event(s), Condition(s) or Action(s)), similar to what is described in the answer to the question about "How to login-redirect users to their profile page on specific days only?".

PS: Using the Rules module to make something happen when something gets "published" should be a no-brainer (that's like Rules lesson 101, which I assume does not need any more explanation).