The thing that has occurred to me about the Rat Park experiment is that while addictive substances can be and are used as a substitute for social interaction, more often, and especially with addictive substances whose use is traditional within the culture in question, using addictive substances is a social activity. And indeed, with alcohol in any case, drinking alone rather than drinking socially is considered one of the signs that the drinker has become an alcoholic.

Now, I've only read a brief summary of the Rat Park experiment, but I don't remember any social drug use by the rats.