When viewing a Contact within Pocket Informant, sharing the contact via iMessage will not work. The problem here is that ultimately this is a bug in iOS itself. We might be able to work around it and we've been researching the problem and trying to find a workaround which may show up in a future release.
Apple provides an OS that gives us a LOT of pre-written components - this is standard across mobile OSes. It is very valuable for OS makers to do this because it provides consistent, prefabricated components that work the same across many apps. It lets us focus on our app instead of reinventing the wheel over and over again. However when there is a bug in that component, it hurts everyone.
In this case when you tap on a contact, you essentially exit PI's code and move on to Apple's code. While we wrote our own editors for appointment and tasks, we use Apple's viewer and editor for Contacts instead of our own. You can see this because the UI slightly changes and is different from ours in some ways. Why not write our own? For a few reasons, mostly because there are some features such as Facetime and Ringtone editing that cannot be done by third party apps, but can be done via Apple's editor.
So when you tap on a contact you open Apple's UI and everything in that window is theirs. Every button press, every action. Its in a black box that we mostly don't control. We could control it if we wrote our own, but then you'd lose a lot of functionality which doesn't make sense. We are looking to see if we can intercept it so we can fix Apple's bug, but again I can't promise anything.
If you're technically minded, you can search on ABPersonViewController for the UI component we use.