Instagram is a good platform for promotion, but maybe it is little bit difficult or hard. If the steemit accounts created on Instagram remains active on Instagram for long time. Also if the campaigns remains on Steem platform to boost Instagram accounts, then this can be easy.
Whatsapp will be great to promote steemit. It's groups system and status system will help us Alot.
Keep up good work, these platforms are very much helpful to promote steemit.