Princeton University Library: Chat page explains hours the service is available. It includes the familiar icons for the chat services in addition to the names. They include a FAQ and an email option. The email option is a form. To cut down on spam they have an image on the form that has to be viewed by the person and the text within the image typed into the form. The chat service is linked on the library's main page (though small in size) but does not appear on other pages, including the catalog, and most notably not on the "Contact the library" page.
Santa Clara University Library: Only available during open hours but hours are not listed, just a link to when the library is open. Other options are prominently displayed on the page, including 24/7 service and telephone service. A link to Ask a Librarian appears on the banner of every page. (did I miss that in Princeton's site?)
St. Charles Public Library: The page explains when the service is available. There is also an button saying someone is currently available live. Other options are prominently displayed. The "I'm online" button appears on the mail library page but not the catalog. Links to online chat are on other pages.