Twitter have an interest in detecting anomalies in their service. Anomalies could be down to user engagement, spamming or technical issues. Regardless of the reasons, it’s something they want to know about when it happens.
To aid detection of anomalies in their time series data they have developed, and open-sourced, an anomaly detection package for R. Their algorithm is based on the Generalized ESD Test and can detect both global and local anomalies. The package is also capable of detecting anomalies when seasonal and trend factors are present in the time series data.