Affiliate marketing is a method of promoting web businesses in which an affiliate is rewarded for every visitor, subscriber, customer, and/or sale provided through his/her efforts. Compensation or commission may be made based on a certain value for each exposure (CPM), visit (Pay per click), registrant or new customer (Pay per lead), sale (usually a percentage, Pay per sale or revenue share), or any combination of them. Merchants like affiliate marketing because it is a "pay for performance model", meaning the merchant does not incur a marketing expense unless results are realized. Some businesses owe much of their growth and success to this marketing technique, especially small and midsize businesses. However, unlike display advertising, affiliate marketing is not easily scalable. Some merchants run their own affiliate programs while others use third party services provided by intermediaries to track traffic or sales that are referred from affiliates. (see outsourced program management) Merchants can choose from different types of affiliate management solutions including: standalone software, hosted services, shopping carts with affiliate features, and third party affiliate networks. Affiliate marketing has grown quickly since its inception. The e-commerce website, viewed as a marketing toy in the early days of the web, became an integrated part of the overall business plan and in some cases grew to a bigger business than the existing offline business. According to one report, total sales generated through affiliate networks in 2006 was £2.16 billion in the UK alone. The estimates were £1.35 billion in sales in 2005. MarketingSherpa's research team roughly estimates affiliates worldwide will earn $6.5 billion in bounty and commissions in 2006. This includes retail, personal finance, gaming and gambling, travel, telecom, 'Net marketing' education offers, subscription sites, and other lead generation, but it does not include contextual ad networks such as Google AdSense. Currently the most active sectors for affiliate marketing are the adult, gambling and retail sectors[citation needed]. The three sectors expected to experience the greatest growth are the mobile phone, finance and travel sectors[citation needed]. Hot on the heels of these are the entertainment (particularly gaming) and internet-related services (particularly broadband) sectors. Also several of the affiliate solution providers expect to see increased interest from B2B marketers and advertisers in using affiliate marketing as part of their mix[citation needed]. Of course, this is constantly subject to change.