Wow... you just explained my experience at Twitter. I went from being a future PhD looking for a fun summer to someone who forgot about grad school and wanted to stay creating "real" stuff in 4th and Folsom for ever.
Suffice it to say, I ended my internship, left grad school (including a fellowship by Google via GEM) and stayed at Twitter.
Get an internship, it can change (or confirm) your vision about a career and your future in general.