Now think of Facebook. With the new 'like' option, you can literally be on Facebook almost everywhere on the web. For marketers, they're unaware of knowing what visitors are doing on sites when it comes to Facebook use because visitors need their ID to log on. So it's either have Facebook's back or deal with being left out of a huge chunk of website's visitors' information.
Advertisers, marketers, and website visitors should think seriously about who they're essentially allowing to manage the web completely before two or three companies take over completely.