I tried a new approach in explaining Pleiades last week:
Are you familiar with "Google Places"?
Pleiades is like that for the ancient world.
The analogy is only a little bit bogus, and I'm going to try explain that now. Google's Places (and Facebook's and Foursquare's) are about linking the concept of being at a store, or venue, or landmark with advertisements, reviews, deals, reservations, media, nearby places, resources (including maps) that reference them – largely for commercial purposes. Resources that link to other resources. Pleiades places, on the other hand, are about linking the concept of studying an ancient city, road, or region with nearby places and references to supporting articles, books, maps, digital editions of inscriptions, and archaeological GIS. These are scholarly, not commercial nodes in the web.
That's not to say that there is no data in commercial location sites worthy of research or that there's nothing in them worth emulating. While I'm not personally interested in "playing" Foursquare, Gowalla, or the like, I think we'd be completely off track if Pleiades didn't help make something like a visualization of Alexander checking in, ousting mayors, and gaining badges across Asia trivially possible.