Why so few users (on WCG)? Below are my thoughts.

Of the globalPopulation, only a few can afford the cost to buy, operate, and maintain a computer. On a globalBasis, the masses consider a person having a computer -- as a 'rich' person. The masses don't care much, nor can they afford to do, what the 'rich' person chooses to engage in: look for aliens, or engage in other forms of concerns that are, well, 'only for the rich'. The 'rich', meanwhile, don't care much for what engages the masses: food, water, and fight against diseases.

Of those 'rich' that can afford a computer, only a few do those concerns that are 'for the masses'. It is so much prestigious among the 'rich' to be seen as engaging in discussions, activities, or projects related to the search for alien life, alien planets, or alien creatures. No such prestige-, and with that, no excitement-, in dealing with cleaning water or fighting cancer. These concerns are 'for the masses'.

Thus, if one can afford a computer, there is a good chance, SETI or MilkyWay, or PrimeGrid, or Collatz will get their share of the crunchingPower in that computer. These sites do not deal with concerns that are 'for the masses'; these sites deal with concerns that excite the 'rich' -- which is the same thing as saying that these sites do not relate to the masses who are interested in the more down-to-earth concerns like food, water, and fight against diseases.

The idea of WCG doing advertising via the F1, as I see it, will not work to increase the count of WCG crunchers. F1 is a passion for racing among the rich, while WCG is not doing 'for-the-rich' concerns like getting to know the largest prime number, or the plotting of the MilkyWay celestial objects in 3D.

The GPU front is the 'savior' for WCG in terms of increasing the chances of attracting new crunchers and/or increasing crunchingPower. The 'rich' after all have been doing it, and they show in force in MilkyWay, SETI, and other popular grids -- crunching via GPU. WCG have been missing the GPU beat, sadly.
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Why so few users?

I'm a student at a UNi (UK O.U) with six hundred thousand students. All have computers. The OU WGC Team numbers just fifty people. No amount of prodding by me or evangelising in student fora gets the numbers up. WGC is interesting on all fronts; humanitarian, social history, nerdy, technically elegant, provides a huge contribution to humanity, yet my student colleagues have no interest. It's less fun than Twitter and Facebook, why bother?

There's also a general fear amongst my chums that anything installed on a computer opens it up to virus attack. Many of them don't know how toembed a URL into an email.

It's so sad, really.

Well there must be a way to get more people on. Someone must have figured it out in 2004. When I look in my team's stats I notice the vast majority of contributors joined in 2004 thru 2006. Only a fraction of the team joined in the years following.
So what was the message that got people to join back then?

I found out about WCG thanks to Watson competing on Jeopardy. After some research I have been actively processing WUs since I signed up this year. Fascinating! Thanks to all involved. Rory