Somebody put an upsetting and demeaning fake craigslist posting about with my daughters contact information. Is this illegal?

Assuming you can prove (or think that in court, you would be able to prove) that the ex-boyfriend posted this, you should have a cause of action against him. What he did, attributing a false factual statement to your daughter, that put her in a negative light, is defamation. Defamation is actionable, or something you can sue over, regardless of whether it is spoken, in print, or online-based internet defamation. An attorney can help you evaluate the strength of your case and your evidence, as well as what you and your daughter may be able to recover in terms of compensation.

However, you will not be able to sue craigslist. Federal law exempts interest service providers or the owners of websites, like craigslist, where the site merely hosts other people’s content (rather than creating the content themselves) from liability for defamation. It seems very unfair—the site gets to pocket the profit from listing demeaning or defamatory material, but doesn’t have to bear responsibility for it—but unfortunately, it is settled law. There have already been many cases involving fraudulent demeaning craigslist postings, and in all cases, craigslist itself has escaped liability.

So if you are fairly sure of your facts, speak to an attorney about going after the ex-boyfriend; but you will not be able to sue craigslist.