Hi,
There are many social networking websites like facebook, orkut, twitter. These websites handling many users at a time. All the queries are processed faster. Which database do they use for building it?
Also they handles for login,user creation,scarps, photos etc. all these must be stored in a database.
How they create relatioships with each table? Will they only link each and every table with ID.

mostly mysql

With all those applications, it has a photo application. How are they saving images? Are they saving in a directory or in a database? Which is the best option

filesystem

filesystem

do you mean filesystem as directory?

directory, folder, hard drive... yeah call it whatever you like, it's outside of the database

You could certainly keep track of the files in the filesystem with the database, but you'd store all the photos and other media OUTSIDE the DB.