Many of the best known Internet companies have focused on allowing people to express themselves which is something everyone wants to do, even if we don't admit it. We do it everyday, in its most basic form, through speaking and writing.
It's no coincidence that the founders of Twitter were previously successful in the area of self-expression when they developed Blogger which was sold to Google. After all, Twitter is just a microblogging service. What simpler way to express yourself than in a mere sentence or two?
Photos and videos are commonly used by people to express themselves in art. (As Hugh MacLeod said, "Art's purpose is to express consciousness.") The key to creating a popular service is obviously to make self-expression as frictionless as possible. Twitter is a leader at doing this. So is Instagram. But with video and podcasting it gets a little more difficult since those media require editing. Apps like Vine, Instagram, and Aviary add quick and simple edit features to refine or spice up the content, but there's a huge drop off in users for anything more involved.
Art vs EngineeringWhile the self-expression aspect may seem obvious in these web sites, it's interesting to take a look at them from a different perspective. Another way to view Blogger is as a web front-end to FTP. Twitter is just a web interface for RSS. While Hotmail was the first big web interface to another underlying service (SMTP/POP), the king of all web interfaces is Amazon Web Services.
So, if you're not the artistic entrepreneur then find a problem where a web front end would be innovative and eliminate a pain point.