I personally think this is getting a little too segmented and pedantic. Someone who hosts their own instance of Wordpress is not necessarily a "pro webmaster." Most hosting companies allow you to buy a domain and hosting services, then setup your own wordpress blog in one or two clicks. Adding plugins to Wordpress is not difficult or a "power user" type of function. The Wordpress admin is basically a web-app itself. I don't see the point in segmenting questions about the limited wordpress.com hosting vs. a self-hosted wordpress instance.
I think these stack exchange sites need to be careful to not segment the communities too much. In my humble opinion, the biggest reason that Stackoverflow works so well is because it's a community for programmers across all languages. You get one large user-base, and they can see questions across all languages, and they can choose which questions to ignore or view. It would not work as well if you had a C# site, a separate Java site, etc. In my opinion the large user base is the key to success. Sure, I could ask this question on the Wordpress-specific stack exchange, but my chances of finding a user that is familiar with both Wordpress and Last.fm goes down, compared to asking in a more general site like this one. If I ask on "Pro Webmasters," the question will probably be closed as off-topic, because it's not related to professionally administering a web site.
I think the question at hand is appropriate for "webapps" because it's regarding a web app (wordpress), which has a web-based admin app, which allows to install plugins related to other web apps without any coding whatsoever. I originally asked this question on superuser.com, and it was closed there, so I thought I'd try asking on the "webapps" stack exchange, but now I'm being redirected to other yet-to-be-created stack exchange sites. I'm losing interest in finding the exact correct home for this question, and I think most users unfamiliar with the "stack exchange engine" are going to be put off by having their question redirected and debated in this fashion.
I guess my main question here is why must this question be closed on webapps? Is it really that damaging or unfit for this particular community?