According to the Delaware Public Benefit Corporation rules it requires a two thirds majority of share holders to change status. If that is held up by the courts it's some teeth but not absolute protection against change.
If they gave over a third ownership to EFF things would become interesting. Don't see it happening though.
Under 2/3rds I wonder what would be best long term? A founder owning at least a third could be a problem if when they died, those shares went to someone who was more interested in profit than stated goals. Maybe to get access to the social network you have to buy a share? Spread out the shares to enough individuals and put in language to prevent proxy voting and require a minimum percentage of share holders to vote to change those stated goals?