The trend at Justin.tv is just the opposite:
What about other similar websites? Maybe personal webcasting is a passing fad? Do the competing sites show a similar downward trend? Actually, no, they don't. As a group the other sites are doing rather well.
A couple of newer sites (uvlog and ivlog) are too small to have Alexa measurements. Two sites went out of business last year (or was it longer ago than that?): Stickam and BlogTV. Stickam simply went away, but BlogTV was acquired by Younow. Here is Younow's chart:
Finally, what about the new home of a number of disaffected JTV users and casters? Let's take a look at Vaughnlive: