Major environmental, social and economic changes threatening the resilience of ecosystems world-wide and new demands on a broad range of forest ecosystem services present new challenges for forest management and monitoring. New risks and threats such as invasive alien species imply fundamental challenges for traditional forest management strategies, which have been based on assumptions of permanent ecosystem stability. Adaptive management and monitoring is called for to detect new threats and changes as early as possible, but this requires large-scale monitoring and monitoring resources remain a limiting factor. Accordingly, forest practitioners and scientists have begun to turn to public support in the form of “citizen science” to react flexibly to specific challenges and gather critical information. The emergence of ubiquitous mobile and internet technologies provides a new digital source of information in the form of so-called social media that essentially turns users of these media into environmental sensors and provides an immense volume of publicly accessible, ambient environmental information. Mining social media content, such as Facebook, Twitter, Wikis or Blogs, has been shown to make critical contributions to epidemic disease monitoring, emergency management or earthquake detection. Applications in the ecological domain remain anecdotal and a methodical exploration for this domain is lacking. Using the example of the micro-blogging service Twitter and invasive alien species in forest ecosystems, this study provides a methodical exploration and assessment of social media for forest monitoring. Social media mining is approached as an opportunistic citizen science model and the data, activities and contributors are analyzed in comparison to deliberate ecological citizen science monitoring. The results show that Twitter is a valuable source of information on invasive alien species and that social media in general could be a supplement to traditional monitoring data. Twitter proves to be a rich source of primary biodiversity observations including those of the selected invasive species. In addition, it is shown that Twitter content provides distinctive thematic profiles that relate closely to key characteristics of the explored invasive alien species and provide valuable insights for invasive species management. Furthermore, the study shows that while there are underutilized opportunities for citizen science in forest monitoring, the contributors of biodiversity observations on Twitter show a more than casual interest in this subject and represent a large pool of potential contributors to deliberate citizen science monitoring efforts. In summary, social online media are a valuable source for ecological monitoring information in general and deserve intensified exploration to arrive at operational systems supporting real-time risk assessments.