Twitter is a popular micro-blogging site, where you can post short messages (tweets) about yourself. But users are starting to see a new type of directed, unsolicited advertising based on their posts. For example, say that you tweet about having an ear infection. This is monitored by someone, and you receive a message from a medical company suggesting that you try their product to help with the infection. It’s an advertisers paradise, because the messages are directed to those most likely to be interested in the product.