It's illegal in the US to advertise that your house is to rent only to particular groups: a "Canadians-only (they're so tidy!)" ad would be unlawful. However, such ads flourish on Craigslist. Rigel Oliveri's checked through the ads and finds that the vast majority of ads noting a discriminatory preference are from folks looking for roommates (rather than evil racist landlords); that the vast majority of those ads are from folks who didn't want roommates with children; and, that the majority of ads expressing a discriminatory race, ethnicity, or religious preference are from members of minorities looking for roommates of the same background.
If ads on Craigslist are representative of the kinds of preferences that are forbidden in other advertising venues, we have a problem. Where the intended effect of the Fair Housing Act is to reduce racial discrimination and protect minorities, the actual effect is to keep members of minority groups preferring to have same-group roommates from advertising same - and to make it harder for non-family friendly places to advertise as such - increasing search costs in housing for everyone involved.
HT: Max Stearns