Since you're asking the community some questions regarding their thoughts and opinions on the old game, as opposed to seeking assistance for a game-related issue, I'll move your topic over to the "Game Discussion" board.~
The demographic of the earlier days was a bit different than it is now. I think one of the features that many would consider a foundational element of the good times were the original maps, which all held vastly different biomes (and offered a nice slice of everything but also felt jarring to jump from a snow map to a tropical island), but there is a lot of discussion surrounding when exactly the game's "golden age" began and ended. (I'm generally hesitant to view the entirety of Feral Heart's progressive days through rose-colored-glasses, because there are always negative undertones that lurk beneath what it is we're given to see, even when things appear to be going so well.)
I know many players joined at a young age (including myself), and as the years have passed and those people stick around and grow up, the "age" of the population matures, as well. (Not necessarily referring to the quality that is often regarded as a positive attribute-- rather as in the kind of behavior and mannerisms that those individuals exhibit.). Having joined in 2012, I was able to see much of this shift, and even found myself being parts of the changes that occurred. My personal observation from the progressive days of the game is that there was a significant variety in terms of how people interacted within the game-- lots of different and random roleplays (we're talking tree and toilet roleplays xD), multiple little "hub areas" in certain maps that would be populated and universally recognized for both a physical distinction and a sociological one (FNAF and Undertale fans hanged around Ficho Tunnel, Hetalia roleplayers would chill in Ascension Island, and TLK lovers made frequent appearences in Bonfire Island), and of course, the chatting that you can still see in the game, today. A lot of the same things took place then as now, just with a different energy and appeal. I would characterize the earlier times as parading spontaneity (there were lots of different roleplays that would be occurring within a radius and pop up sporadically). This contrasts to more condensed geographic that is prevalent now. The Grounds is what has been adopted as the generalized area where everyone interacts, aside from the fleshed-out roleplays that usually take place into the other maps like Cherika Valley, where users may want to distance themselves from the on-going chatter of the busier areas. The way users interact in-game currently has everyone involved in the same general area, instead of people populating different areas. I would describe the present atmosphere as being patterned-- or otherwise, (positively) consistent and (negatively) monocultured. In quite a few ways, the old "feel" of Feral Heart still lingers around, but it has transformed, as the years have gone by and some people have left, with new players joining.
Yes, there were more players. Feral Heart was a successor to KovuLKD's prior game, Impressive Title, so it already had some buzz going around, which naturally gathered attention for the game's release. (There were so many players attempting to connect to the server on the first day that the server crashed and Raz had to pull it down. cx) As time goes on, numbers go down when new content isn't added and your product isn't updated; that is normal, and for a game like Feral Heart, it became a lengthy process to actually implement changes to the game since the owner did not leave behind the game's source code.
I think what I miss about the old era is the complex and varied interactions, and the energy that went along with it-- albiet, that also brings about a sense of chaos. More wacky stuff means lack of consistency and predictability. In comparison, there are less crazy happenings now in the game, at least on a large scale, but I think that is largely due to the playerbase of the game and the types of mindsets occupied by the individuals who serve as the majority of what you'll see when stepping into a map. You could argue which setup appeals to you more, and I think a lot of it has to do with what you specifically are looking for in a game. Each "age" (previous and present) offer a different commodity to what a player might want in a game-- the earlier times give you a fun world to pretty much walk into something that you hadn't expected to see a minute ago, and the current times give you a chance to listen in to what kind of topics are being talked about and find with people to chill with. (Both "ages" have both qualities, but I would say this is where the "atmosphere" has evolved.)
I sort of took a more broad, people/demographic-centered approach to this topic, since I personally believe one of the greatest influences in the change of dynamic took place within the people who played the game. Other factors contribute to the change, such as the lack of updated after LKD's departure, recreation of the world, etc., but the shift had a gradual effect and could be seen slowly as the years went on. Thus, the defining lines of where the "Golden Era" ended is quite tricky to say-- factors of it still remain, while others were not long to stay. This speaks to me as a change in player dynamic. I think the other comments summarized their points well on this subject. I hope I was able to offer something of value.