I would say this community is very active for how many people are in it. Also, it isn't the QUANTITY of the community but the QUALITY. The quality of the Construct community is impressive from what I've seen. Very friendly people.
Besides the technical specifications of Construct being better than Game Maker (some of which have already been mentioned), you will always OWN your games with Construct. With Gamer Maker, you have to buy a new version if you want those new features. If you don't buy the new version but still want SOME (only some because you don't get everything if you don't pay) of the new features, then you get a big banner that said you used Game Maker. For me, that was a big deal. It didn't feel like I'd actually own my game.
So Construct is faster, stronger, and is getting improved all the time while still being free as in freedom AND price. GameMaker is old (older DirectX and lack of features), slower (VERY slow), and makes you pay again and again if you want anything improved. Sure, Construct doesn't have as many finished games to show off, but there will be, oh yes, there will be!