Just a suggestion to possibly improve the UI. Might not be to everyones preference. Theres "Chats" and "Contacts" text on the bottom and the top which is a bit of a duplication. I like the text titles on top as they round of the design quite nicely but I think the bottom should change to icons. Also instead of clicking the buttons at the bottom to switch between your chats and contacts make them into indicator icons to tell you where you are and implement a swipe (left/right) to switch between them. I did a rough mock up below.