For me the most efficient way to get FP is to play memory - it's pretty easy, you can get max. 1000 FP for submiting the high score after the game, once a day. Polls are also nice, but it takes a lot of time (and clicking), at least for me.
Knuffels also are nice. You get 1000 FP for every 10 levels, and they level up really quickly at first, as it's always a case

Also, there's a chance to get FP while digging. And it's higher the lower the level of your knuffel is, because you get the chance to get FP only if you don't get any material. Usually you get only 10 FP, but you can get also few other amounts, up to 1000.
This chance was bigger some time ago, but at that time knuffels were more expensive and there weren't any trial knuffels, so you didn't get your first knuffel for free... So I personally think it's better that way.
As for forum, yes, it isn't really active anymore. There are ocasionally some new posts, but there isn't much going on. Well, maybe except of the hangouts. I don't frequent them anymore, but for sure they are the most active part of the forum. There's also a Caravan, it usually makes the forum a little more active, since everyone wants to get unique items you can get only for donuts, and buy only during the Caravan. But it also depends how many people are around at that time, with some free time on their hands.
I personally love Firn's art - that's the main thing that keeps me here for all these years.