People Who Say They Have “a Facebook”
Facebook is the name of the website. Each member’s page on Facebook is not referred to as that person’s Facebook, but their Facebook page. Similarly, someone could properly say that they have a Facebook account. This also applies to MySpace, for what few people still use that site.
I also hate people who call Wikipedia “Wiki”. Wikipedia is a wiki, but there are thousands of other wikis on the internet. It’s not a term unique to Wikipedia. Pretty much any website with interlinkable pages editable by users is a wiki.