Used mainly on social networking sites such as Facebook and Twitter when someone writes a status that is completely dull, boring, mundane and usually of absolutely no interest to anyone.
To give someone an ASBU is like an online version of an ASBO (Anti-Social Behaviour Order), except it's mainly used for boring shit.
Status: Sarah Drearyton is walking.
Comment: Wow. ASBU!
Status: Bob Shootmenow is eating chips.
Comment: For the love of God Bob... ASBU!