Pidgin will probably struggle with this because Fb Messenger is quite a moving target. You can currently send audio, video, photos, stickers, and payments. Also it supports audio and video calls. Not to mention messenger platform which enables developers to build just about anything else.
Plus, the dynamics around group chats are tricky. Messenger shows when people are typing and what point everybody has read up until. You can rename group messages, leave them, add people, mute/disable notifications, or mute for a period of time.
Without good compatibility or fallbacks for all of this behavior, Pidgin users are in for a bad time. Most will probably go back in frustration and pin messenger.com in a browser tab.
Except that even with a limited feature set, Pidgin has one advantage: it integrates with a lot of networks. It's the same with IRC: Pidgin's IRC support is mediocre compared to real clients like XChat but I still used it. I'd only have to launch Pidgin to log into multiple IRC networks, MSN, two Google Talk accounts and Facebook. It has a single logging mechanism, single interface, it's not a website, there are no ads and I can install/update it using apt-get.
It would be nice if Pidgin would build in all the features you mentioned, but I'd be perfectly happy if it just connected to my chats and allowed me to send and receive text messages. Typing notifications would be nice-to-have, but this is all I ever use anyway. Not such a bad time for me.
Agreed, connecting to other networks is huge. But the other features might be more important to others. Personally, I would rather use multiple clients than entirely miss the stickers, photos, payments, and group chats that get sent my way.
That said- I'm sure pidgin can go a long way by implementing sane fallbacks for these features.
Plus, the dynamics around group chats are tricky. Messenger shows when people are typing and what point everybody has read up until. You can rename group messages, leave them, add people, mute/disable notifications, or mute for a period of time.
Without good compatibility or fallbacks for all of this behavior, Pidgin users are in for a bad time. Most will probably go back in frustration and pin messenger.com in a browser tab.