Firstly, I think it's a slightly delicate task trying to get Hubbers away to other sites as Hubs can't contain referral links. One can include RSS feeds into an original Hub and hope they join via a link in a blog post or in a Xombyte - just more difficult to know who's joining without a direct ref link.

Also, Hubpages does (at least according to Alexa - mmm...) get more traffic than Xomba but I also can't make much money out of it and, like ngureco, now tend to make short articles giving max space to adsense and nothing to amazon.

One other comment above I agree about, and that's to give extra mileage to those articles on the revenue sharing bookmarking sites.