Why big internet companies (Google, Facebook, Twitter, Yahoo, etc) going so much open source ?

If you were ever wondering why these big comapanies (Google, Facebook, Twitter, Yahoo, etc) going so much open source. There are various factors that contributed to this attitude of these new corporates. These factors are Cost of training, Delivery Time & Quality of delivery and ultimately assurance about replicating infrastructure at thier level needs lots of capital. Now each fators in detail, 

Cost of Training - Any big comapany goes into phases of attrition then new hires as norm not exception. So, they want new people to at least familiar with technologies they are working with, if not the whole process.  

Delivery Time & Quality of delivery - For these big internet brand names delivery time and Quality of any service or feature and addressing any critical issue in short span of time are crucial for their brand name. They are earning because of their brand name otherwise their are many pirates are out there. So, they need developeswho can write quality code in short time. And, these two things increase over time of developer association with technology used. Therefore, this is most important factor why these companies want developers to know about what they are working upon.

Assurace - BIG Internet companies are more about scale and process rather than trade secrets and softwares. they know their most of the client side code can be easily be duplicated. So, they concentrate on not on how to protect that but concentrate on scaling each product to a level where others can't do that much of scaling. Even, scaling to that level is also possible but that is certainely very capital intensive. So, they are almost assure that their scaling cannot be challenged in near future.