Maybe not so bad,when booking hotels for a trip within Turkey two weeks ago got fed up when every advert led back to Booking.com seemed as though they had a monopoly.
They don't have a monopoly but they are the biggest and own a lot of travel & leisure oriented sites. Priceline (the company's original site), Kayak, Opentable, rentalcar.com, Agoda are some of the better known ones but there is a complex network of minor brands and sites that all ultimately lead back to booking.com.
Expedia is another company with lots of different brands - Travelocity, Orbitz, Hotels.com, Hotwire, Trivago, CheapTickets and eBookers plus a whole range of lesser known brands and sites.
Basically, the online booking market is controlled by two companies - the only other serious competitor is possibly Opodo which is owned by a consortium of airlines but it appears to perform as well as the airlines themselves, i.e. poorly!
JF