Marijuana is currently legal for recreational use in the states of Washington and Colorado, with Colorado having just opened its first cannabis shops this month. However, weed businesses won't be able to advertise their wares on social media for the time being. Google and Facebook don't plan on changing their policies barring the promotion of weed use, even in places where it's legal.
The risk is too high, no pun intended
According to GigaOm, the National Cannabis Industry Association, a trade group that lobbies for weed-related businesses in Washington, claims that it attempted to pay Facebook to promote stories covering marijuana but the site refused since it's policy doesn't allow it. Facebook told The Verge that, while it is willing to place ads that promote marijuana advocacy or legalization, the risk in running ads that promote the sale and use of the drug itself is "too high (no pun intended) for us to consider at this time." A Facebook representative did say that the company is always reviewing its policy decisions, but doesn't anticipate this matter being addressed in "the next few months."
Google's own AdWords policy disallows the promotion of drugs and drug paraphernalia (even if it does allow products that reference them). However, like Facebook, it did state via email that it is always reviewing its policies, though they haven't recently changed.
It should be noted that Facebook and Google can geo-target ads for specific demographics, but, given that — unlike alcohol — marijuana is still illegal on the federal level, they may be exercising some caution before they change their policies.