The age of pristine tweets is over. Twitter has changed its terms of service to allow ads on the site, Reuters reports. The microblogging site boasts 45 million monthly users and has been seeking ways to make money on the site. Analysts are split on whether the move is a good one. Skeptics say companies won't want to juxtapose their brands with unpredictable and potentially offensive content, while the optimistic point out that users get hooked on social-networking sites and burn a lot of time on them, making sites like Twitter attractive to advertisers, particularly if preferences are tracked.