I am working on a project for 3 niche stores built on magento (using magento's multi-store function) that each get roughly 50k unique visitors a day. The sites don't currently have a blog or forum or any social networking aspects.

Would like to add a cms to each site that can be centrally run and would like it to take over the front end content from Magento. Also would like it to maintain an online blog/publication of sorts with videos, articles, and the like with privileges to edit the content given to a dozen or so people with different privileges. Want to add a forum to each site that is fairly robust and to possibly add some social networking aspects down the road, so extandability and available plugins/mods in each cms is important.

Other than shared login between the forums,blog/publication and store, would like to be able to integrate some content from the forums and blog/publication into the store as well.

After researching this a bit, I am inclined towards Drupal, but I haven't found any modules to integrate it with Magento. Also, since the blog content will be done by about a dozen nontechnical people, I want something that is very easy to work with. Lastly, since the site gets a good amount of traffic, speed and security are very important.

What CMS would you recommend integrating in this context? Deciding between Drupal, Wordpress and ModX. Also Plone as well.

Thanks.