IMO has said that they are now testing a new feature on their mobile apps. Contacts in the search result list would be sorted on the basis of how frequently you contact them.

This comes handy for users who have way too many contacts in their instant messaging list. Imo said:

Currently, when you click on the “People” search bar at the bottom of your screen to find the person with whom you want to chat, you are presented with an alphabetized list of contacts. However, once you click on a contact to start a chat, they will move up on your search list depending on how often you talk with them. The more you talk to a contact, the higher they will appear.

These changes are now available in the IMO beta app for Android. They would roll it out to the stable edition on both Android and iOS devices.