Battery drain due to contacts (I think)

Hi.

I have noticed that my standby time increases even with the screen off, and battery runs out quicky.

This only happens if I have running on the background an app which uses contacts (like whatsapp, tango, line....)

I have an iPhone 5 with ios 6.1 and I try everything (Restore with iCloud backup, restore as a new iphone, hard reset, delete all contacts, unsync contacts from icloud...) but nothing happens.

What do you think? anybody has the same problem?