Usability
Interface 
User-friendliness/complexity of the tool´s interface
Straightforward, intuitive.
Time efficiency Time needed for mastering the „how-to“ of the tool and a production of a task
High.
Interface language(s) The number of languages in which the tool is currently available/adaptable to
English only.
Respect for privacy Does the tool require registration? Who can see my data? Can I remain anonymous?
Medium. Diigo collects user data such as e-mail addresses, RL, IP address, browser type and language, and the date and time of request. It also saves annotations made by users on its servers. However, it does not record users’ browsing history or share users’ data with third parties. Diigo hosts third parties that collect cookies, such as Google Analytics. There is little information about security in Diigo’s privacy policy. Diigo complies with the GDPR. See more here.
Saving work for future use Possiblity to save your or others´ work for reuse/reproduction
Yes, with an account.
Tailorability (If so, how?) Possibility of the tool to be used for a range of different activities (e.g. to what extent you can organize a project assignment with a central activity based on this tool)
Yes, it allows bookmarking any website and has different functionalities.