Yes you must use the hashtag share2steem in either the Instagram or twitter posts. So you can tweet and instagram as much as you want, and then just use the tag where you want to share it to steemit. This is handy for us folk in the @worldofphotography group who can then enter contests simply by posting from Instagram to Steemit without logging onto a pc, transferring pics from camera / phone to pc, and then posting on Steemit. Saves us so much time.