Years back when I was working with a digital advertiser, we did a sweepstakes offer across mobile devices. I started out having to deal with the legalese, then when the media director left in the middle of the project, I had to finish bringing it to market. I can tell you based upon my own expereince, a sweepstakes is a double edged sword.

Let's put aside the lengthy (and costly) legal requirements of running a sweepstakes offer and focus on what goal trying to be accomplished and if a sweepstakes offer will help achieve that goal.

Consider a sweepstakes like an incentives based lead generation ecosystem. In the advertising game, incentives based leads are all known to be dog shit because, while you get registrations and numbers (which Steemit already is getting in droves), you do not get engagement and genuine product interest. People will sign up only for the free iPad, cash or (in this case) STEEM.

The goal should be more about engagement than registration, so the sweepstakes should have some barriers of entry, such as either engagement based upon contribution (such as only people who have written a blog entry within the past week are eligible), or engagement based upon curation and sifting (such as only people who have both voted and commented within the past week are eligible). The key to getting the quality lead/registration/action you are looking for is to understand how to incentivize actions which create the continued desired outcome.

A sweepstakes should not necessarily be a regular thing, because it will then become the reason people engage, rather than the reason you are hoping they will engage.

My $0.02 based upon my own experience for what it's worth.