Craigslist routinely takes down discriminatory housing and job ads and have been doing that at least since I started using it around 2005.
And I generally agree. The authorities should be going after the perpetrators, not the neutral platform.
The party which should be pursued is the one posting the post which goes contrary to law.
I'm ok if FB or CL proactively root out those posts, but I'm not so sure I see it as their responsibility to do. It's good oversight but still I feel the people who should be punished are the agents of the wrongdoing.
So, no, I don't see them as an accessory to the crime.
That said, the companies/individuals doing the actual discrimination and or illegal solicitation should face the consequences of their actions. In this case, I'm not condoning companies discriminating on the basis of age.
Not calling out Craigslist --simply illustrating that regular tools meant to be used responsibly can be abused to accomplish nefarious ends.