class User
  include Mongoid::Document

  has_and_belongs_to_many :tags, inverse_of: nil
  field :email
  field :social_profile
  # ... other user fields
end

class Tag
  include Mongoid::Document
  # ... tag fields, nothing fancy here
end

User.where(social_profile: nil).only(:email).each do |user|
  user.social_profile = lookup_service.load_profile(user.email)
  user.save!
end