Well, I think they would probably use different engines for the specific job needed. But I will say now, they most likely use a WHOLE server just for a MySql server and nothing else. If you are planing on making a database heavy application then you really must have the MySql server on a dedicated machine.

check out http://www.mysqlperformanceblog.com/ you will have to do some looking and some reading but provide a lot of really good information for performance tuning and/or what you can expect from both engines.

I've checked the link but I'm still confused.
I've googled this question a lot but there is no direct answer.
Everyone says that Myisam is good when you have read operations only or update operations only. and Innodb is used when you have a mix between read and update. but what if you have many read operations and some update operations? It's right that you have read operations more than update but still.

I'm still confused. Please help

If in doubt use PostgreSQL

Now that is an accurate response.