I was in a similar boat, Grasscutter. Here’s what I learned and I hope it helps.
Choose any realm in a popular cluster. A cluster is just a group of reams which are connected in the same time zone/region. You will see the same players across all connected realms in the cluster. Connected realms allow you to interact with other players as if they were on your realm (you can talk to, group up with, join the same guild, send mail, etc.).
Fortunately, connected realms are more true to Classic. They are not the same as Cross-Realms, which were introduced during Cataclysm, I believe, and made other players that were not on your realm literally phase in and out around you (like ghosts). It added to the erosion of the community feel because you’d often never see the people in your party or even around you in cities again. That is not the case on connected Era realms, fortunately.
Anyway, there are basically four popular clusters of connected realms. One PvE and one PvP cluster exists on the West Coast and a corresponding pair on the East Coast. I can’t speak for the West Coast (or the PvP East Coast) but the PvE East Coast cluster of connected realms (Ashkandi, Mankrik, Pagle, Westfall, and Windseeker) usually shows about 15-50+ people in Stormwind and Ironforge (depending on the time and day). I’m on Pagle and I know at least half a dozen or so guilds are actively raiding. There are additional social guilds, if your just looking for casual and/or leveling.
Here’s a list of connected realms:
Horde and Alliance numbers seems pretty even on my cluster.
Whisper some people about their guild and join their Discord. Once you’re in a guild, I think you’ll feel more at home and you’ll have the best opportunity to run dungeons. (There is still some activity in the LFG Channel but guilds are your best bet.)
Just yesterday (even on a SoM realm) I was talking in Guild Chat more than I have in a long while. My guild plans to live on and move back to Era (and/or to Wrath, to give people the option to play both).
I will still play Wrath for a bit just to satisfy my curiosity. But I think it’s going to be like drinking too much Red Bull or eating too many Hot Pockets. And, as we know, kids (and probably most trolls) prefer Red Bull and Hot Pockets. So I plan to keep playing on Era for a long time to come, which is like enjoying a nice home-cooked Pot Roast.
Here’s a decent list of guilds on the PvE East Coast cluster (thanks to Servius):