Portal for ArcGIS 10.3.1 passes an incorrect parameter 'longUrl' to the bit.ly sharing/URL shortening API wherein the portal web adaptor name is referenced twice only when the web or item is shared with 'Everyone'.
Do not configure <a href="http://bit.ly" target="_blank">bit.ly</a> sharing with Portal for ArcGIS.Only share up to the organization, but not everyone.Manually send the GET request with the correct URL parameters after sharing with everyone. The correct link is available in the final sharing configuration.