Even if this app is entirely above board (and the apparent lack of a data protection registration makes me wonder if it is), then what is the point?
Yes, in theory, it enables us to approach the MPs more easily, but what about in practice? In practice, it'll probably go the same way as other dedicated social networks (such as Microsoft's Yammer), in that it'll become a place where we can get a little valuable information and a lot of noise. It is likely to ultimately become just one thing on a long list of things we need to check for messages, and another app we can leak data to.
At the very least, if the MPs aren't responding to existing methods of communication (email, fax, phone, snail mail, twitter and facebook), then adding one more to the list isn't going to make them more likely to respond.