Before anyone asks: it's not a lot of shrugs. Hard to believe, I know, since all these bozo fitness routines would have you think your traps are only to bring your shoulders to your ears.
1) Three motions, three sections. Your traps are broken up into 3 sections or heads. Each of these sections is activated at higher levels with a corresponding motion (this being said all of the motions activate all the sections but there is an optimal method to train each section). First is the Upper Head of the Traps, which is optimally trained by elevating the shoulders. This is where your shrugs come into play. Second is the squeezing of the shoulder blades together for your Mid-Head of the Traps. Shit like rows hit this pretty good but if you wanna get a little thicker around the spine do prone shrugs. The third is the Lower Head of the Traps which has its function in stability of the shoulder joint. So overhead pressing and deadlifting will develop this area just fine but if you're finding that the top of the dead and top of the OHP feel unstable you can always try overhead holds, waiters walks, deadlift holds, or overhead shrugs.
2) Load Dampening. Your traps can be thought of as a brake for your arms. They are the muscle that prevents your arms being torn out of socket, more or less. Movements, where you have to catch the bar at your hips (think the return of a clean to the ground) the shocking load of catching weight at your hips, is incredible for traps. Funny enough the guy doing shit shrugs with 600lbs might be on to something with this. I found this out as sort of as a byproduct of not owning bumper plates and enjoying the Olympic lifts. I cant just drop my steel plates from overhead, I have to return them to a Deadlift position and lower them down. This is why old school Olympic weightlifters had big ass shoulders (that and the Clean and Press, but that's another story entirely).
3) Time under tension. These bad boys can handle shit tons of weight for long periods of time too. Movements like rack pulls and farmers walks will always be superior to shrugs because the load is just so much higher and/or its held in position longer. Farmers walk is an excellent trap developer because of the small movement of the weight while walking causes an unstable environment. If you've ever done a set of overhead press with an Earthquake Bar (bamboo bar or whatever) you'll know the strange soreness you get a day after; kinda the same ballpark. You're essentially dampening small inconsistencies while under load, that's a 2 for 1 deal.
4) When to train them and what about Face Pulls? Yeah man do them. I sprinkle in smaller moves at the end of my training often as a little circuit (as you can see in AE or RN). Most the time I will hit traps with my pull days (back/deadlift) and push days (delts) with some sort of something. Upright rows, OHP, and lateral raises all smash your traps and delts if you're doing them right because those muscles work in tandem. Much like how rows, carrys, and deadlifts hit your back and traps. It's all about the Trap Synergy my guy.
Traps stabilize the shoulders, pinch the shoulder blades, and shrug the shoulders. Use heavy compounds that create a lot of time under tension and/or lots of eccentric load. Use smaller moves for higher reps or with pauses. Train traps directly or indirectly with back AND shoulders.