Ok so I was considering trying this game out because it has some interesting features that I haven't experienced, plus its free. My only concern is about the community.

Are the servers populated well enough? Which server would be best for newcomer to start in? Are people generally social within this game?

I realize that there will most likely be immature people, but thats nearly unavoidable.

all of the servers are definetly populated enough, and this is a very social game. I've heard praise from people that Mari is the "best" server. Mari is one of the three oldest servers for this game alogn with Ruairi and Tarlach. I personally play on Alexina and enjoy it there. Alexina has been around for a couple years and is the newst of the four servers. If you wish to join Alexina feel free to add me =P I'm total level 1k+ IGN: Akeal

You wont regret trying out Mabinogi.

it's not that Mari is the "best" server (though of course i think it is lol), but mari was considered by many to be the continuation of the original "nao" beta server. ti's comparable to Scania for MS. there is a huge player base on mari that has been playing since beta (myself included), but that doesn't mean we don't welcome new blood  :)

feel free to join us~

Each server, surprisingly, on Mabinogi, with it's low amount has many players. Though, I have some doubts about two. Alexina has a ton of lag due to neglecting by Nexon, but has supposedly been fixed with Generation 13 (you'll figure out what this is soon enough. It basically means - major update). Mari is full of the most players, so that might mean overwhelming levels. This is why I suggest Tarlach or Ruairi when you start playing.