Atmospheir is simple to set up and start using. You can opt to create a new account or log in via Facebook. The software syncs with your iOS address book and Facebook to form a contact database. Contacts are presented within the app as "cards" that you swipe to navigate.
Atmospheir is more than a unified address book -- it's also a messaging app. You can send and receive texts, phone calls, photos and videos using the app. And you can send messages and receive updated address book information from contacts who don't use Atmospheir. (That's a good thing, because none of my roughly 1,500 contacts use the app.)
If you know people who do use the app, it gains more functionality. For example, a "Nearby" feature uses Bluetooth to find other Atmospheir users who are within range of your phone, so you can quickly exchange contact info cards when you're at a conference or other event.
Unfortunately, Atmospheir isn't optimized for iPad, and it's only available for iOS.
If Atmospheir doesn't cut it, check out FullContact, a robust contact management app for iPhone and iPad. The service is also available as a Gmail plug-in for the Chrome browser. You can add contacts from Google, as well as Facebook, Twitter, LinkedIn and other social networks. I appreciate FullContact's Gmail plug-in, which provides information on people who've emailed me in the past, along with icons to quickly create calendar invites or initiate Google Hangout chats. FullContact is free for up to 5,000 contacts, and a $10 monthly fee bumps up the limit to 25,000 contacts and adds other perks.
Another option is Cloze, an Android and iOS app that lets you import contacts from Gmail, Microsoft Exchange, Outlook.com, Yahoo and other accounts. The premium Cloze Pro plan ($20 a month, or $160 per year) lets you link Evernote notes to your contacts, it and accesses your call history, which syncs up with your contacts. (You need to give the app access to that history.) Cloze also offers a 14-day free trial for its premium service.
Assuming you don't need a full contact management system, such as Salesforce, each of these apps is worth a look.