how to create api and use creating Web API like Twitter API or Facebook API?

Hi,
I have developed a website as example.com where all the employees of many companies will come and write their opinions and comments on their employers with their scores such as good, bad, excellent, etc. and this data needs to be shown on example.com and on all the clients websites with those actual data with the scores e.g. good, bad, excellent, etc. for example I have got 100 employers in the database and all the employees of those 100 companies will come and write their scores on a single website (example.com) and then my role is to show the real data related to that particular company to their website. And I am not allowed to show the x company’s data on y company's website. Meaning thereby, XYZ-employer should get the real data related to XYZ employer and not of the ABC employer and ABC employer should get only ABC related data. And then employers need their data on their respective websites.

So here I will have to manage the data for them and the employer being the client of example.com should get the api code for integration with their respective websites. So I need to develop such functionality where the employee can write their score either on example.com or on the employer’s website but then the scores should be updated on example.com and the employer’s website (clients) simultaneously. This application is supposing to get the scales very soon. That is why, I want to compare all the possibilities considering the heavy and bulky database of example.com because very soon there will some more functionalities. So I am not sure how to go ahead, so tell me if you can what are the possible alternatives for doing so. Please suggest me ASAP.
Will it be like creating Web API like Twitter API or Facebook API? which shows the actual likes or tweets related to that log in account on website as well as on client page of facebook and twitter.
Please explain. IT's VERY VERY URGENT.

Tags: ASP.NET