Well, I play in Ruairi server, with the IGNs: Himself (Giant), Saerah (Elf), Limes (Human). AnnabelleMarie also plays on Ruairi server. Much of the SOS Brigade guild here at ggFTW plays on Mari though. I can't attest to their activity since I'm not on Mari, but Annabelle and I play pretty frequently.
If you're starting on a new account, make sure to make a human with that first card. When you make a human, you can use that character to get a free card for an elf and a giant.
There are no job advancements, although recently the 'destiny' system was implemented. Destiny system allows you to pick one of four 'destinies', which have minimal bearing on the game (it makes certain stats grow faster, and makes skills related to that destiny train faster). Example: Warrior destiny makes all melee combat skills grow twice as fast, and gives 0.5 more strength per level-up.
The game itself is actually very active, we just don't have an outstandingly large community for Mabinogi here at ggFTW.
As far as a basic startup is concerned, for all characters there are a few skills you *should* train as soon as possible (I say *should* because nothing in this game is really required of you, it's all up to your preference). You should try to train up Windmill, Refining, and Wild Animal Taming as soon as possible. The reason being is that, as you progress further through the game, these 3 skills become exponentially harder to train (a lot of skills get more difficult, but these are the cream of the crop for late-training difficulty).
Windmill and Wild Animal Taming get more difficult because of an in-game factor known as CP (Combat Power). As you train skills and level up, your combat power rises. As your combat power rises, it becomes more difficult for you to find enemies within a certain range of your combat power to train your skills on.
As for Refining, since it *requires* failures in order to train it, it becomes more difficult to train as your dex rises. For giants it is okay to take your time raising this skill, because they don't have many skills that raise their dex outside of life skills (refining is one of these life skills), but for humans and elves, it is recommended to do refining as soon as possible. I also recommend doing it soon because the market for buying ore fragments is at an all time low (at least in Ruairi, unsure of the other servers).
Anyway, if you have any questions, feel free to message me at my giant, Himself, on the Ruairi server.