Are they asking you in person or sending out a Facebook invitation/post asking you to like their page?
If it is the latter, keep in mind that Facebook makes it very easy for anyone to cast a wide net inviting all of their FB friends with one click. It is impersonal, and as such doesn't warrant a response. I get a bunch of these requests routinely and generally ignore them.
However, if this is something that comes up face to face I agree with the previous suggestions that you can say you'll look into it later.