I have been using Facebook Messenger on Android for 2 years. The application is quite good in many ways and has also some limitations as well.
SMS Alternative :
Facebook Messenger is a very good SMS alternative. It uses just your data load which you already have.
You can send messages to your Facebook contacts and others having Facebook account or even Facebook pages.
You can send Pics, audio and video messages without any major charges.
It runs smoothly in background and remains connected to Internet andgets connected easily even if you dont have high speed Internet.
Too many Options :
It gives various options like it informs you about status of messages Sent, Delivered or Seen and time of action as well.
Video calling :
Video calling is very nice and if your data speed is just more than 100 Kbps ( 2G) . But at high speed Internet Video calling is awesome. It consumes around 1MB/minute for video call.
Voic clarity is not good at 100Kbps speed. Better you have Internet speed better voice you will get. Moreover it also depends on the Internet speed of other side also.
VOICE CALLING :
Voice calling is a very good option for low speed Internet. It consumes less data and is better on 2G network.
Connectivity :
App gets connected very easily and instantly shows new messages if any.
Some Limitations of Messenger :
It consumes a lot of space in your phone usually more than 250MB on android 6 and more than 150MB on android 5.1.
As it occupies more RAM and Memory it is not suitable for Low end phones like having 0.5MB or 1MB RAM. It consumes tremendous RAM during video call or Exchanging Pics or more messages and makes software of such phones crash.
?it often shows oneself among the Active Friends. Which is confusing.
?Sometimes you are unable to remove certain contacts. Even if you are not Connected via Facebook and Sometimes shows people in active friends who were removed long ago.
?you can’t delete call history of a person at once, you need to delete each call one by one.
?If someone doesn’t pick up your call, it auto starts recording voice message.
?Sometimes you can not see some of your Facebook Friends in messager contacts.
?Emojis and Gifs dont load faster
Sometimes it shows old profile pic of your contacts who have changed profile pic long ago.