Seems like all they'd have to do is add value to advertisers using the existing ad-word engine. The plan is always the same for any internet holding - build up user base, then invest heavily in the advertising engine to add value for the true customer, advertisers. If you listen to FB conference calls, it's always "we want to build the service to over a billion users, then we'll work hard to create the ad engine to get advertisers as much value as we can."

Same way as Youtube. 1. Make advertising massively more efficient by using adwords 2. Cut IT costs dramatically by leveraging existing infrastructure and knowledge 3. Make search more effective to increase user engagement 4. Continue to use Twitter's firehose to improve Google search results about new items and therefore drive revenue to Google's flagship product (they have an agreement in place with Twitter to do since 2015 so but if MSFT buys TWTR that's not going to last and suddenly Bing gets an advantage in generating relevant search results on rapidly emerging issues).