While few brands struck social media gold on Super Bowl Sunday, Esurance was able to break through by putting a digital twist on one of the oldest marketing moves in the book: a sweepstakes.

People are still talking -- and tweeting -- about the insurer's post-game ad, which dangled $1.5 million in front of viewers. The rules were simple: Tweet "#EsuranceSave30" for a chance to win all the money. The phrase refers to the fact that Esurance says it saved 30%, or $1.5 million, by running the spot immediately following the game, rather than during the game.

As of late morning on Monday, 2.39 million people had entered the contest, including 200,000 entries that streamed in within one minute of the spot airing, according to the company and Leo Burnett, which handled the campaign. Meanwhile, "EsuranceSave30" continues to trend on Twitter globally, and the campaign has already generated 1 billion impressions, according to Leo Burnett and Esurance. All the while, the brand's Twitter follower count has grown from 8,900 to 155,000.

But the campaign carried risks, namely that people would hijack the hashtag. As CNET pointed out, there have been some offensive tweets using "#EsuranceSave30," including some bashing the company, which is owned by Allstate.

Esurance says it has rules in place to disqualify inappropriate tweets from winning the prize, which will be selected after the entry period ends at 4 a.m. eastern time on Feb. 4. "It was something we anticipated and there's always going to be a few bad eggs," Nancy Abraham, Esurance's VP of advertising, said in an interview. "The good news is the response has been overwhelmingly positive." The rules "say we can disqualify entries if they are inappropriate or offensive or they have used derogatory language or information," she added.

Does that include tweets that simply criticize Esurance? "It depends on how they are positioned and I think we'd look at it on a case-by-case basis," she said. The winner will be picked at random and announced during ABC's "Jimmy Kimmel Live" show on Feb. 5.

Tweets included groveling -- one person pledged to name his son "EsuranceSave30" -- as well as just plain silliness, like one tweet noting that the prize could pay for 7,168,458 Chicken McNuggets.

The company purposely kept the rules simple to emphasize its brand positioning. "Every piece of this was about communicating our strategy of being a company that's designed for life in the modern world -- for those people who live their life online who want to do business with a company that is fast [and] efficient," Ms. Abraham said.

Even the campaign, which included paid digital support in addition to the TV ad, came together relatively quickly. "We pulled this campaign together in about two months … which really is a testament to our efficiency and the agility of our teams," Ms. Abraham said.

Outside of flooding twitter with the Esurance name, users are not asked to do much else. For instance, they are not directed to a phone number or web sites for insurance quotes, as so many other insurance ads do. The goal was to raise brand awareness, said Darren Howard, Esurance's VP of marketing. "Super Bowl Sunday was a great place to do that," he said, by putting the company alongside other big brands that advertised around the game. "It was a nice elevation for us from what we have been doing over the past two years," he added.