DOJ is a bit  off as this space is not a Duopoly or monopoly try whackopoly from a-y (maybe a-z but I only know up to Yelp hence the a-y)

From Amazon Patents to Yelp and a whack in between the review niche space.

Amazon

Constantcontact

Demandforce

epinions

Exacttarget

icontact

Kulist

Marketo

Metacritic

Retrevo

Responsys

Vertical Response

Viewscore 

Vocus

Salesforce.com

Shopwiki 

Yello