Social Media Melting Pot

Why is it that everyone sends all their content from other social networks to Twitter?

Someone checks-in on Foursquare, I see it on Twitter.
Someone likes a YouTube video, I see it on Twitter.
Someone posts on Pinterest, I see it on Twitter.
Someone posts on Tumblr, I see it on Twitter.

You get the idea.

Wouldn't I be following you on Foursquare/YouTube/Pinterest/etc. if I wanted to see all that stuff? Am I the only one annoyed by this?