Backup wallet of single account As far as i know that online wallets as Blockchain.info is a Bitcoind instance that gives each user an Account and each account related to multiple addresses through JSON-RPC calls, how blockchain.info backup wallet of individual account ? i imagine that all accounts are stored in a single wallet.dat ! or does blockchain.info has wallet.dat for every account ?
Nobody really knows how it's implemented, other than the developers of the site, but I doubt they keep a single wallet.dat file. They don't store the keys of each address so it could very well be that they don't even use bitcoind. 

They could use their own implementation which would allow for better concurrency since they have lots of users accessing their wallets at once.

You can build a bitcoin wallet however you'd like. It doesn't have to be a wallet.dat file. You could even put the information of the transactions in a basic MySQL database (which is good at concurrency) and have something like PHP process and send out the transaction, and you'd have a wallet represented within a MySQL database.

To answer your question, I'm sure of course they do have proper backup routines of their database, just like any other website.