On July 21, the
swe subdomain was added for Software Engineering, making
swe.stackexchange.com a redirect to
softwareengineering.stackexchange.com. Now that there is a shorter URL, this should be the URL that appears when using the share button on posts.
Doc Brown points out that one use case for this is when including links to other questions or answers in text length-constrained fields, such as comments on the SE network. Comments are limited to 600 characters, so eliminating 16 characters from the URL automatically would give commenters extra space.