Looking at Steam's Stats page, Half-Life 2 is currently sandwiched between Plants vs Zombies and X3: Albion Prelude. But this weekend, if the Call For Communication Steam Group pull it off, Valve's own game should be somewhere near the top ten. The protesters are hoping that the surge of players will send a message to Valve: “we want to know what's happening to Gordon Freeman.”
I think it's a smart way to prove a point: reversing the flow of information back to Steam's Seattle HQ, using Steam's servers to bring that message back to the developers. It's not pushy, it's not demanding, and, as someone waiting for the completion of that story as well, they have a point.
The entire trilogy of episodes was supposed to be completed and released by 2007, and if Valve have decided to do other things for the time being, that is fine; all that we ask for is a basic response on the matter, and to let fans know whether or not the current story arc is scheduled to conclude at another point in time.
In addition: This message is in no way, shape or form attempting to rush the development of the Half-Life series; in fact, most members agree that Valve should take the time needed to deliver a complete and polished product.
In a way, Valve's method of story-telling and development has led to this. They left the second episode on a cliff-hanger, and it's been nearly four-and-half years since Episode 2's release. But there's an incompatibility in how Valve work and what fans sometimes expect. The company has a democratic way of making things happen: things get made if enough people want it to be done. That's why Left 4 Dead 2 landed so quickly after the first, that's why they're making Dota 2. Aside from when Gabe Newell tells everyone what to do, it's what the 300+ group of people vote on that forms projects.
So will it have an effect? I'd expect there's definitely an emotional resonance from what the group are attempting somewhere in Valve. I have no idea of the state of Episode 3 or Half-Life 3, although I was shown a very tiny glimpse of a whiteboard of information at Valve's offices in 2008. It might be it's in bits and pieces, or it might be six months from being released, but ignoring an organised public plea for recognition by fans in a subject as emotive as Half-Life has risks. A PR plan usually doesn't take into account feelings.
If I were a Valve employee, I'd be patching a message into Half-Life 2 right now.