I love me3. I think the reason it's in decline is because of all the cheaters. It's really annoying when you put on your best items for a platinum game and someone gets infinite rockets. If you see anyone sporting the striker shotgun or hear anyone fire
off all their rockets at the start of a match you might as well back out or you're just going to be bored for half an hour.
The decline in Mass Effect 3's multiplayer was and is caused by several reasons.
1. Unpatched cheats/exploits that allow players to breeze through even the top level difficulties.
2. The repetitive and unchanged nature of the multiplayer.
3. The lack of new content. Bio has stopped with their new characters/maps so it is getting increasingly stale.
Also, make sure you have ALL of the free multiplayer map packs downloaded else you will find very, very few people playing as you must have the same packs as others to even connect to them. Hope this helps!