“Facebook blocked Twitter from accessing your friends list way back in 2010; Twitter turned around and blocked Instagram from accessing your Twitter friends in July, after Facebook bought it (also Tumblr, Google, and LinkedIn); and Instagram just blocked its photos from showing up in Twitter.
“The reason everybody keeps blocking everybody else isn’t simply a matter of minimizing the growth of competing services in any way possible. It’s because all of that data about who and what you follow is incredibly valuable—invaluable, really, since it’s the core of their business model: Use that data to deliver highly effective advertising. Facebook’s social graph is its secret sauce; Twitter’s interest graph is its magic fairy dust. Both are largely based on who you follow and who’s following you. So they don’t want to give the other guy a peek.