I think that some of the appeal of the on-line answer sites is the ease of not having to find the right person to ask and being able to get those answers 24/7. I also believe that part of the appeal of on-line answer sites is that a person can ask any question about anything while being anonymous.
AskMetafilter - I checked this one out first and it seems that most of the questions I see being asked are more of a personal nature - not educational. You also have to be a member to ask questions on this site, which to me makes it less useful in a school setting; and the fact that membership has to be paid for makes it unusable for young students.
ChaCha - I didn't even check this one out because cell phones and portable devices are not allowed at our school and this site is for mobile devices.
AllExperts - I like this site. I think this one could be useful to the students. I like that it has a homework help link that is divided into catagories that are broken down enough for the kids to be able to find the right area pretty fast. I don't know that they will care which person they get the answer from, but being able to ask more than one "expert" the same question is beneficial.
WikiAnswers - okay, now I guess I won't ever suggest this site to any student. I went to wikianswers.com (clicking the link on the More Things site) and asked, "Who was the 12th president of the United States of America?" and the first hit I got was about ovulation and it got detailed....not student friendly. Everything listed below my question was related to pregnancy. Not exactly what I was looking for.
Yahoo!Answers - this site is user friendly. Type a question on the line and you are given all kinds of options for the answer. I think that this site would be great for older students (middle and high school) but offers too many choices for the younger (elem) students.
Askville - this site doesn't appear to be as direct to your answers as I would be looking for - although it doesn't seem to have so many social questions.
Answerbag - this again seems to be a social networking question/answer space. I did ask the same president question and although I didn't get an inappropriate answer - I didn't get the answer I was looking for either.
Mahalo - once again - seems more social networking than educational question/answer.
folkstown - lots of twitter going on here. I think that for research and educational questions this is not the place I would send a student.
Snappyfingers - this too is not the site I would send students to for research.
Most of the questions I was seeing on the answer sites listed where not the ones I hear in the media center. Sometimes I get questions of a more personal nature, but those are usually from students who know me outside of school. I can't say that I would be comfortable handling those kinds of questions from my students; and I can't say that most of the sites that seem social network oriented would be of too much help to them in an educational setting. I don't feel that these sites are competition to the reference library.