How do you know when your followers are online and you have the best chance for your tweet to be seen and retweeted to get the maximum exposure? Use Audiense, a free tool (used to be SocialBro). Audiense also helps you to find influencers – folks with a lot of followers. The idea is to get noticed by them and get them to retweet your tweets. See the 2nd video.
How to Find the Best Time to Tweet with Audiense:
Note: you need to have at least 100 Twitter followers for it to be effective.