So after coming back from the NACIS conference I've been looking at cloud mapping again. At the conference Mapbox, CartoDB and Leaflet and ArcGIS online were getting a lot of mentions. Compare that to searches from Google trends:
My interpretation is that:
The non-experts are using/interested in ArcGIS online or Google Maps Engine
The experts are interested in the others.
Cloud mapping is on the up (as far as search terms go anyhow)