How can I tactfully avoid "friending" my mom on Facebook?

I’ve been dreading this for a while, and it’s finally happened. I got a friend request from my mom on Facebook. For those of you who are familiar with me here, you know I’m not a teenager who doesn’t want to be spied on. I’m 35 and a parent myself, but use Facebook a lot to keep in touch with friends from high school and college.

My mom lives quite far from us, but we talk on the phone often. She has some emotional or psychological issues, and I try to keep a certain boundary with her in order to avoid all of her drama. I don’t want to hurt her feelings, and have never said or done anything to make it clear that she actually drives me crazy. I really don’t want to have her posting her nonsense on my Facebook page. Is there a way to tactfully avoid having to friend her? Any advice here would be much appreciate. I love my mom and do not want her to be hurt or unhappy, but I also have to maintain my own sanity and peace of mind. She is extremely difficult to have a relationship with. I’m not being petty, I just don’t want to write an entire novel about her problems.

Thanks in advance!