While nothing has ever lived up to an old Mac app called Fluid for me, there are a few good options for creating Single Site Browsers (Mac apps dedicated to a single web service) these days. Flotato is the fastest and lightest weight one I've seen, while still offering niceties like automatic mobile view and dock badges. It can create a new app from a service like Hulu or Facebook almost instantly, and the browsers it runs are lightweight. Plus it has the automatic benefit of siloing your use of a service from your other browser activities.
