Applum did a study over over 50,000 pages and over 1,000,000 updates and found that Facebook is keeping 3rd Party posts from being as available in your feeds.
The chart above shows what kind of interaction a post made directly in Facebook gets compared to a post coming from an outside party.
Facebook actually, quasi admitted this:
“We’re focused on ensuring that users see the highest quality stories in News Feed,” a Facebook spokesperson told us. “As part of this, related stories are typically aggregated so users can see a consolidated view of stories from one app. In some cases, we work closely with trusted partners, such as Preferred Developer Consultants, to test new ways of surfacing stories, and gather feedback to improve the Platform experience.”