diablo 3 online

Is the game any good?

No one's playing campaign anymore, there is no point. Everyone's grinding adventure mode on season. Grind grind grind, booooring

It may be that most players are doing Rifts and Adventure Mode instead of story mode.

This is an awesome game, but all the content is focused for the end-game (lvl 70). Everyone is playing content from the last expansion (Reaper of Souls) and the last patch. I suggest you to purchase it if it's not done already (if you intend to play with other people). Also, nobody plays the acts (story) anymore but rather the new mode that came with Reaper of Souls, the adventure mode. You'll find people at Tourment VI and higher difficulties but not a lot below. Again, this game is focused on the end-game content.

Yes - you are doing something wrong ;) Doing Campaign is not worth it for any veteran players - it takes much more time and offers much weaker rewards. But to actually play online you will also need Reaper of Souls expansion - without it you cannot hit max level, and 99.9% of playerbase are long sitting on max level already ;) My advice would be get Reaper of Soul, play through all 5 acts of campaigh at least once to get the story, achievs and get a grip of basic enemies, their abilities, osses etc. Then go play Adventure mode on lvl 70 - that's where most of players are sitting and playing.

I know that all my Diablo 3 loving friends stopped playing it to focus on World of Warcraft: Legion. This could partly be one of the possibly many reasons why you struggle with finding other players. Also you'll generally struggle to find groups on campaign as people level up and grind in Adventure mode.