It kind of comes down to how well you can defend yourself from it being called a DOS attack (follow politeness standards and robots.txt), from violating their copyright (generally not problematic if you don't distribute the data), and from violating their terms of service (this is key in the case of twitter and reddit, carefully read their TOS).
However, the scraping of public information like in the case of tweets or reddit posts is the less problematic part. It's when you distribute the data or aggregations of the data that it could be problematic to use scraped public information.
