Console games are never popular on Twitch tbh, even if it's Battlefield or CoD. That said, the official forums are pretty dead outside of some old-timers and a few obnoxious trolls. The community never recovered from the move to the shitty PS community boards anyways, the old ND forums were the best, too bad Sony ditched it for their garbage unified platform and essentially killed the community's official, most populated meeting place.
Reddit is the most active, surprisingly, but at this rate it won't hold much longer. They need to release co-op and make it F2P as soon as possible.
I think that /r/unchartedmultiplayer is doing pretty well considering it's a new community.
Yes, the forum update killed the official PS board. Not just for Uncharted, but for just about every game on there. The official community forums are dead. That's kind of a shame, but honestly it's often better to use outside platforms anyway, places like reddit give the community more freedom to create a platform for discussion that suits the players.
/r/unchartedmultiplayer isn't that much less popular than /r/factions was to be honest. Generally speaking Naughtdog games are overlooked, and underdiscussed. I think part of this however is that for many players there isn't all that much to discuss. Uncharted games lack a strong competitive scene so there's no reason to discuss in-depth strategy, they lack loadout and team comp flexibility compared to games like League or even Call of Duty, and generally speaking they do not have many complex mechanics that might spark discussion normally.
Even /r/Battleborn is vastly more active than the Uncharted community and I can assure you battleborn has less significantly less concurrent players, even when both platforms are combined. Sometimes the strength of the games surrounding communities does not reflect in-game activity. While the leaderboards still suggest that quite a few people do play Uncharted 4, it may simply be the case that they aren't very vocal, or passionate about the game. Honestly I think that a lot of people that do hop on simply do so because they bought the game for SP, and decided to occasionally hit 'multiplayer' while they wait for their next game.
In comparison, Battleborn isn't a popular game in general, but the community that remain are very passionate, the games systems incredibly in-depth and the community picked the game up with a the intent of playing with other players, hence the very active community in comparison to Uncharted's.
I love the UC4 multiplayer. I play Team Death match almost every time I turn on my PS4. It's oddly relaxing while being tense at the same time. Is there anymore multiplayers like it on PS4?
The best feeling is healing somebody on your team with the mystic or medpack. I LOVE that feeling.
Your only other options for similar games, and by similar I mean third person shooters are Star Wars Battlefront and The Last of Us. The Last of Us is far slower paced and serious in tone, so it may not have the same atmosphere you enjoy in Uncharted 4. Star Wars Battlefront is pretty chill, pick up and play nonsense. Most of what you do feels relatively inconsequential, and the game isn't balanced very well, but if you just like running around blowing people up in a third person perspective, then it's a comparable experience.
Ultimately though the reason many will stick with Uncharted 4 for the time being is the sheer fact that there aren't any other decent options on the Playstation 4. Naughtydog woudn't get away with things like shipping this with peer to peer, on a 15 tick rate, if there were other games competing in a similar space. Regardless of its issues, Uncharted 4 has the best moment to moment gameplay feel of any of these games listed (except TLOU, which is comparable, and shares many of Uncharted 4's merits, and weaknesses).