Monetizing

Google.

And you can't make actual money off of a fanfiction archive or the companies, authors, ect who actually own them will sue you.

Re-consider doing that Xarah, Jan is right, fanfiction is illegal in the first place.

http://www.chillingeffects.org/fanfic/faq.cgi

Then again I believe harrypotterfanfiction.com advertises and hasn't been shutdown or run into problems.

It's a fine line. People do not have a right to write fanfic but whether you will get in trouble or not depends on how you are making money from your site and what the purpose is. If you're merely raising funds to cover costs, it'll probably be ok. If you think your site is a great way to make an extra $100 a month, then it becomes more convoluted .  If you are making money from ads when people visit your site, that's fine, but if you are charging them for membership to read and/or post, you're likely to face problems.

It's up to you, really, but be careful. Chances are if you get shut down, it will be specifically for hosting fanfic, but if you are charging money, the owners of the copyrights could try to collect damages.

It also depends on the fandom.  JKR's publicly stated that fan fic's okay within certain guidelines.  Nora Roberts/J.D. Robb has publicly stated don't touch my stuff.  Who's more likely to sue you?

I advertise on two of my archives, only to help pay for hosting.  I haven't run into any legal issues, but I'm mostly advertising official merchandise anyway.

Honestly, you probably won't make enough to cover your hosting, let alone make money. Most ad places won't think you have enough traffic unless you have 1,000 unique hits daily, and the ones that will take you will give you a crappy rate.