iOS: When it first launched, Cobook was one our favourite address book apps for iOS. Development eventually stopped when it was purchased by FullContact. Now, it's back, and it works very similarly to the original app.
Like Cobook, FullContact syncs all your contacts across various services, adds profile pictures from various sources and syncs any changes. The mobile app is based on the FullContact web app and works the same way. Once you give FullContact access to your contacts (yes, you'll have to do so if you want to use the service), it then digs around and links up that info with Google, Facebook, Twitter and Instagram. With that info in hand, you can check in on any of your contact's various social media profiles right from the app itself. The FullContact app is free, but unless you pay $US9.99 per month, you're limited to one sync per day and a maximum of 5000 contacts, which shouldn't be a problem for most of us.
FullContact (Free) [iTunes App Store]