Recently someone told me that items shared on ArcGIS Online could not be discovered via a Google search - but exactly the opposite is true. Anything shared publicly in ArcGIS Online is searchable using popular search engines like Google and Bing, though depending on how results are ranked your match mileage may vary.
For example, searching for usa tapestry segmentation will currently show the first ArcGIS Online matches at 4th and 5th in the search results:
The URL endpoint for shared ArcGIS Online items is actually from the arcgis.com website. The surefire way to search for publicly shared items is to use the site: keyword that Google search supports. Entering something like site: arcgis.com followed by a search string will search for that specific string at arcgis.com and return the publicly shared ArcGIS Online items that match:
Google search will typically only return a subset of what you can find directly via an ArcGIS.com search. Why? I'm not entirely sure. So for best results go directly to ArcGIS.com, but clearly the myth that public ArcGIS Online items can't be searched via Google is busted!