In response, sellers and buyers of sex have adopted a wide range of tactics designed to avoid apprehension. The spatial mobility which is afforded by modern communications systems, such as mobile phones and pagers, and by modern forms of transportation, such as taxis and private cars, has severely reduced the ability of police to determine exactly who is engaged in acts of solicitation. Prostitutes have also begun using the internet, in particular instant messaging software such as QQ, to attract customers. In 2004, PlayChina, an online prostitution referral service, was shut down by police.