Marijuana is becoming legal for medicinal purposes very quickly. I am personally a medicinal marijuana user and it has worked better than any other medication I have ever used. If Facebook were to delete these kinds of posts what would that solve? Nothing, at all if you ask me except violating our freedom of speech and upsetting people. Medical Marijuana does make a difference in people's life, like I stated earlier I am a user. People have the right to post what they want on their Facebook, it's their profile after all! If you don't like certain things that people post, simply don't view the material, or delete them as a friend so you don't have to see the posts anymore. We are in complete control of what we view on Facebook. If Facebook deletes these kinds of posts, then they may as well start deleting prescription drug posts as well..
Considering that medical marijuana is becoming legal in an increasing number of states and has been proven to provide medical benefits to patients, it would be a blatant violation of freedom of speech rights. It also seems to overstep Facebook's role to police what people can and cannot post about, especially when that topic is legal and accepted in the users state.
I see no problem with medical marijuana posts being allowed on Facebook. Medical marijuana is technically legal and it's a person's own business to post whatever they want to post on their own Facebook. There are most likely much more horrid and graphic posts on Facebook than something about medical marijuana.
We are protected by the First Amendment to have the right to free speech. There are some exceptions to free speech, such as harmfully slandering other individuals, but posts on medical marijuana do not fall into those categories. Facebook should not act as a Big Brother deciding what topics can be posted and what cannot be posted unless it is a topic that is not protected by the First Amendment
Since the question refers to posts and not ads, I'd be fine with medical marijuana posts like some of the news stories I've seen referring to treatment for cancer patients or gravely ill children. I think it's ok as long as it's clear that marijuana is used for medical purposes, and not presented as a recreational or fun drug.