you can not set Facebook messenger as default messaging app in windows phone 8 . you need windows phone 8.1 which will arrive sometimes in this summer (May-June) , till that time you can enjoy native Facebook chat in messaging hub :smile:
that option will be removed in windows phone 8.1 because it lacks many features and users were having hard time using this. Facebook messenger is great app for social lovers and for people who wants to stay connected to near and dear one . it has stickers , last seen time stamp and much more :smile:
The removal of native Facebook messaging in 8.1 is a terrible let down. It was a feature I used quite often. Thus, I will use FB messaging much less now.
I've been using only Facebook Messenger app since it came out, because it allows me to do much more than the native app, and it's quite well made. Before that came out it was definitely nice to have all messages at one place, but I don't actually mind that much about they getting rid of it in 8.1 now
I really miss the FB messaging in the native app, but this was an expected move, especially since FB messenger, my FB massages stopped coming thru, and I couldn't connect to FB or Microsft messenger at all.