In the era of Big Data the opportunities of discovering knowledge from social big data increase with the risk of privacy violation: during knowledge discovery, the risk is the uncontrolled intrusion into the personal data of the data subjects, namely, of the (possibly unaware) people whose data are being collected, analyzed and mined. Anonymity in generic sense is considered a chimera and the concern about intrusion in the private sphere by means of big data is now in news headlines of major media. Nevertheless, big data analytics and privacy are not necessary enemies. Sometimes many practical and impactful services based on big data analytics can be designed in such a way that the quality of results can coexist with high protection of personal data. The magic word is privacy-by-design. This promising paradigm suggests to integrates the privacy issues in the design phase of a system or application. Privacy intrusion jeopardizes trust: if not adequately countered, they can undermine the idea of a fair and democratic knowledge society. A trust-based and privacy-aware digital ecosystem is becoming unavoidable for creating a new deal around the questions of privacy and data ownership, empowering individual persons with control and transparency on own personal data and knowledge.