It's easy to build a mailing list by using Twitter properly. I know one example of a 60,000+ email list built from a single Twitter account in around two years

It's easy to build a mailing list by using Twitter properly. I know one example of a 60,000+ email list built from a single Twitter account in around two years (2007-2009). Messaging those who follow a competitor is just the smallest fraction of the techniques available for using Twitter for business, and one of the least effective. It doesn't scale well, for example. Also, by analyzing data across multiple social networks, a much wider variety of effective, non-spammy marketing possibilities emerge.