You need clear goals that fit into a hierarchy, with little goals that build toward more meaningful, higher-level goals. Here you are, tracking the footprints of some animal you haven't seen. Then there's the question of feedback. Most Web sites don't very much care what you do. There's also the ability to challenge.
Competition is an easy way to get into flow.
Actually, they're probably more critical. A flow experience has got to be challenging. Anything that is not up to par is going to be irritating or ignored. Your interest in flow came out of your work on the psychology of creativity. Realize that change and downtime are important.