You can place ads at all companies. There is no dispute between any of these companies.

You can promote your Hubs on free Directory sites.The more you promote, the more visitors will visit your hub and having a chance to earn.So post your hub for free Directory atleast 10 per day. So its easy to get more visits for your hub. When ever you create a new hub just start promoting by Free classifieds sites.This is more useful for many things. Many net users are posting regularly for free classifieds to get regular earnings.Trust me posting free Directory also gives you good visitors for you created hubs. No duplication no dead links included.