Django for enterprise web application
performace when number of DB transactions increases.

Not a Django problem, really.
You can have a lot of concurrent Django sessions via Apache and mod_wsgi.  All of them will share a common database.
Therefore, this is entirely a database problem.  You should be able to configure enough Apache/Django sessions that your database is swamped.

OR Mapping included inside Django is a good choice or should I implement them.

Yes.  It's a really good choice.
Until you can prove that the ORM is your bottleneck, use it.
As you scale up, you will rework your database, your cache, and other architectural features.   Since the ORM has a cache (as does your database), you rarely have performance issues here.  
You can.
But most of your performance problems will be downloading static media files through Apache.
I wanted to know is Django a good choice for a big web applicatin(Social Network)? More specifically, I need some suggestion on performace when number of DB transactions increases and I want to know is the embedded OR Mapping included inside Django is a good choice or should I implement them.
Thanks