I'm not a fan of the ads but like the others say, when you accept and use a free product there are concessions you have to make and advertising is one of them.  Most messengers have ads and have had them for a long time (Apple seems to be the exception to this though, although FaceTime isn't a full fledged messenger it is a full fledged video calling app that is ad free although you can argue that you paid for the app with the premium price of the hardware).