You must know that for acts with big demand - Gaga, for example, there are people and sites who buy the tickets on the first day and then resell them for double the price. Sometimes, this doesn't work: look at Miley Cyrus' Bangerz Tour or the Latin American leg of the BTWB. But if there's enough demand to:
a. Sell out the venue on TicketMaster
b. Have the tickets from resellers cost more than the tickets on TicketMaster
it means that the tour is doing well.
U2 never gets these headlines, because there much less interest in rock acts on sites like these and because they do stadiums, which take a whole lot longer to sell out - including the so-called 'touts'.

Some artists simply fail to sell out, see Katy's UK leg:
smaller arenas, yet she fails to sell them all out.