Growing. National player of the year a few years ago (Michael Tagliaferri) is a friend's nephew who went to UNC. He was from the Bay Area. Also, take a look at where some of Nick Meyers' players come from... there's a bunch of British Columbia guys on the roster (traditionally they all carry cannons -- Tre LeClaire is from BC, for example).
There's also a west coast based company (Adrenaline) that does lacrosse apparel and runs college recruiting tourneys. Their New Years tourney in SD was the tourney that my son played in a couple of days after the Rose... They are doing a lot for the visibility of the game out there.
So it is out west... but the adoption of lacrosse at the D1 level is being impeded by T9, which is puzzling to me as it's an opportunity to add 1 for 1 men's and woman's scholarships (note, fully funded men's program is 12.6 'ships per team of ~45 players).
We play a niche sport that we borrow from the Iroquois. That's why you see such a heavy concentration in the NE. In Canada it's also very strong in the Toronto area (proximity to the Iroquois Nation), but covers the entire country pretty well. Interestingly, the national sport of Canada isn't what you'd assume. It's lacrosse _and_ ice hockey. When the ice melts in Canada they play a version called Box Lacrosse that is pretty much hockey on feet. Played inside an ice rink and the ball is allowed to bounce off the boards as a puck in hockey. Suffice it to say, for as watered down as our Field Lacrosse has become (due player safety), there's no such concern in Box. I lent one of my STX titanium shafts (it's an old shaft that is now know in lacrosse circles as the 'lead pipe') to my kid's teammate after he broke his shaft over another kid's shoulder in a box tourney. Handed it to him straight... got it back bowed. I straightened it out by hanging my fat ass 220 lbs on the end of it while it was held in a vice on a shelf in the garage. Think about that. Stick holds a 220 lb man and this kid bent it over some other kid's back. And yeah... it left a mark.