I agree that getting 100s of people to link to LessWrong with the anchor text "rationality" is unlikely to provide much of a benefit (though, hey, it might -- search engines are a big black box), but LessWrong is a reasonably well-trusted site (2k backlinks, most of them quite high quality, see here); having 10s of links (and given how much emphasis Google is meant to place on anchor text at the moment), it could give a substantial boost at the margins.
IMO, I think a better question to ask is how many people are searching for the search term "rationality"? Seems like a weird thing to search for.
This is a breakdown of Less Wrong's recent new user traffic, data sourced from the Less Wrong Google Analytics account.
67% StumbleUpon
16% Google
5.4% Reddit
3.6% Hacker News
3% Harry Potter story
0.7% Facebook
0.3% Overcoming Bias
4% "The Long Tail"
The 16% for Google is artificially high because many of those hits are users that are using Google as an address bar by searching for Less Wrong.
So we get an order of magnitude more traffic from Stumble Upon than anywhere else -- sometimes thousands of new users a day. Stumble Upon has been Less Wrong's biggest referrer of new users from the beginning of the site. That was surprising to me and I suspect it is also surprising to you. Some of our very best users, like Alicorn, came from Stumble Upon.
What can you do?
In order to get maximum Stumble power, you can't just stumble LW articles and only LW articles. You need to use Stumble Upon for a minute or two every now and then and vote up or down the random links it gives you. I know, it's annoying, but what are a few dust specks when we are talking about saving the world?
Thanks to Louie for help with this post.