I previously wrote about Backend as a Service and server-side options and scaling for startups. One service and mobile app I admire and use is Evernote. Their server side is very quick yet serves over 20 million users.
Many mobile server-side services are essentially the same. They have authentication, sync of data and seaching. Evernote has solved most of these problems. The Evernote Techblog explains their architecture, how they use Lucene and MySQL and their indexing system.
Evernote are open about what they are doing because they know it takes a lot more than a technical solution to create a product. Such solutions have to be implemented well and have to be promoted, marketed, sold and continuously updated and supported. Technical matters are a very small part of the equation that results in a successful company.