If you are looking for a light weight simple instant messenger for chatting with your Facebook friends, myChatDroid is the best choice for you. It was known as FBChatDroid formerly.
It is a very small application of just half mb which makes it usable on low end Androids too. As I said this application is just a simple application, it may not be give you a perfect chatting experience. But the developer, Mr. Bruno Mendola, is very sensible to the users’ mails and very much eager to fulfill their needs & correcting teh bugs.
myChatDroid uses Facebook API authentication to connect to your Facebook profile. So it doesn’t require to create any further sign up any other IM sites. It logs in to your Facebook profile via Facebook Connect by entering your original Facebook login id & password.