I thought you were talking on the icon next to the clock.
Sorry, I misunderstood, but yes, as Cookie said, there's no need to keep it open, you can close it down, and if you still want to be able to see your contact list, add your contacts to your desktop by right clicking your contact on your list, choosing Plus! and selecting "Show On Desktop". Although, Im not sure if that will help anything when it comes to memory usage
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I hope that helped,
Truly