In general, the safe choice is to open a link in the same window. I can use the middle mouse button to open the link in a new tab if I want, or I can just click on it, read the contents, and press Back. If you make the link always open in a new tab/window, my options are more limited, and I have to close the tab/window when I'm done, then hunt through my tabs to find your site again and continue reading.

I say "in general", because there might be specific use cases where a new tab/window makes more sense. Maybe you expect the user to want to keep both windows on the screen, or maybe you expect the user to spend a lot of time or drill down deep on the external page. It's hard to say without knowing more, but for a regular social news site, my personal preference is for same window.