I actually disagree with most of what TxMarc suggests.
The keywords and whatnot route will just get folks to come by and take a look see at the individual blogs. That'll get folks to the individual blogs but that won't get them to sign up, create their own blogs, and use them.
Also, as I suggested up above, the hosting and/or blog market is too high priced to even attempt PPC. You've got companies with very big bucks at the top of that market.
I took a look at this page and will make a few suggestions.
1) You're advertising for a competitor. See that wp.org link in your meta widget? Replace it with yours. Ditto with any wp.com links you see. It's in the meta widgets and many themes.
2) Link in the footer. Please a link in the footer of your theme saying that the blog they are looking at is hosted by your site. We actually have a mu plugin that rotates between about a dozen different messages (ie "free blog hosting" "get a free blog today" etc) with a link back to your main site or even a static landing page explaining your site, why the link is there, etc. The multiple messages gets it noticed by Google. ;)
3) Advert widget: Make an advert widget with a 150x150 pixel advert for your site and tell folks it's there for use. Don 't force anyone but suggest it's use. Some will and some won't.
4) Banner across the top. You don't do this but some folks run the admin bar 100% of the time along the top, even if the visitor isn't logged in. Add a banner or text link there back to your site.
5) How your docs? Can you find the help section? Do you even have a help secition?
6) Is it too hard. Log out, clear your cookies, and see how easy it is to create an account on your site.
There's other ideas but most of them involve some serious coding. We have one site that runs a month affiliate program that gives away upgrades. Another one awards for well written posts about the site promoting it.