When choosing a platform for business-critical systems, it's probably not a good idea to go with the "flavour of the month" solution.
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Drupal is mature yet evolving and has a rapidly growing user base. The number of live Drupal installations has grown considerably in recent years and currently exceeds a million. Growth continues at around 25% per year (source: drupal.org).
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Economics: Drupal is a very good solution for many common web applications and it's open-source - no licence fee. But a zero licence fee is only part of the story - Drupal facilitates a RAD approach, offering fast prototyping and reduced development costs.
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Another indicator is Acquia - a company specialising in commercial use of Drupal, co-founded by Drupal's original creator Dries Buytaert, and which attracted $7 million investment capital.
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Google search for Why Drupal
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Drupal case studies and main site