A practical approach is to host the panorama yourself and use a dedicated panorama viewer that supports multi-resolution tiles. From the answers, krpano is a commonly recommended tool for this: you run it on your own web host and publish the pano in a viewer designed for very large images. Multi-resolution viewing is important because it loads only the detail needed at the current zoom level, which makes gigapixel files usable on the web.

Another option mentioned is Microsoft Photosynth, which can host very large panoramas and provides a web viewer, though the answers are uncertain about whether it accepts panoramas created outside Microsoft’s own stitching tools.

If your main goal is just sharing the file rather than an interactive zoomable presentation, a file-sharing service such as Dropbox may work, but it’s not really a full gigapixel viewing solution.

So for publishing on your own site, the best fit from these answers is: use your own web hosting plus a dedicated viewer such as krpano.