Can I say "Do you have a facebook"?

Can I say "Do you have a facebook" to ask if someone has "a Facebook account"? I know it is not grammatically correct. I just wanted to know if people say that or not.

People do say it, but that doesn't make it right or that you should repeat it. People will probably understand what you mean, but it sounds wrong to me.

Ask instead:

• "Do you have a Facebook account?"
• "Are you on Facebook?"
• "Do you use Facebook?"

And note Facebook should be capitalised.

Finally, the website Facebook was named after the face book or facebook, a university directory of names and photos. The original question could possibly be misinterpreted to refer to these.

It's grammatical, but unclear. The addition of account removes any ambiguity.

It's nothing to do with grammar, it's simply about the meaning of the word.

I am not familiar with "Facebook" used in that way, but it would not surprise me at all to hear it.

Similarly, on the Wikipedia help desk, I have often encountered people using "a Wikipedia" to mean "an article in Wikipedia".

Languages change as their speakers and writers innovate (whether consciously or unconsciously).