When you append site:domain.com operator to your query, Google restricts the results to webpages in the given domain. For instance, the query screencasting site:labnol.blogspot.com will find pages about screencasting within this website while the query screencasting site:com will retrieve results from all website that have a .com prefix.
Similarly, if you use the site:domain.com operator with no search keyword, it will give an approximate number of webpages from that domain that are indexed by Google. (see illustration)
During BigDaddy (Google indexing software upgrade), Google discovered a bug was introduced due to which the “site:” query didn’t work with hyphenated domains (like site:www.paris-hilton.com) or when you add a trailing slash (like site:parishilton.com/)
Google has acknowledged the issue and the fixes will be rolled out in the next few days.
Google has also confirmed that the site: operator bug has no connection with webpages being dropped out of Google index.