How to use Google to search over multiple sites

Google has a lot of useful advanced search features.  One of them is the site: filter, which allows you to specify a specific website or domain to limit your search to.  Conversely, you can use -site: to exclude certain sites from your search results.

You can also use the :site filter to search over multiple domains or sites.  The OR operator is used when you want to do that.  For example, to search for "ducks" on Facebook and Twitter, you could type a query like this one:

ducks site:facebook.com OR site:twitter.com

And you would get results similar to this:

You can combine the site: filter with other filters (allintext:, filetype:, etc.) to narrow the scope of your search even further.