I think the answers to this question depend on two related factors: age and mobile lifestyle. There are exceptions, of course, but I suspect there's a strong correlation.
The iPad fills a niche between smartphones on one hand and notebook computers on the other. If you're someone with both, you'll find yourself using one or the other less when you add an iPad to the mix.
Those who use their laptops/notebooks for serious computing tasks have a difficult time substituting an iPad for that functionality. Such folks tend to be older (using their computers for business, for example.) By the same token, such folks are also less likely to be heavily involved in social apps than their younger counterparts and to be out and about less often. Add to that the fact that the older generation appreciate the larger screen of the iPad and the device that gets less use is the smartphone.
Conversely, those who have less use for heavy duty computing, more involvement in social media, are less home or office bound, and have younger eyes have a more difficult time giving up the convenience of their smartphones.
I've been watching this phenomenon for some time (being in the older, less mobile, more computer-reliant group) and I've found my smartphone is used for little other than phone calls. At the same time, my laptop (and my desktop system for that matter) continues to get as much use as ever.
