I have been doing some work on Personal learning Networks (PLNs) recently and Pearltrees is a good example of a component of a PLN: a way of connecting and interacting with learners with similar interests and a way of making new connections and finding new interests. George Siemens's theory of connectivisim underpins the notion of PLNs - stressing the importance of informal, connected learning facilitated by technology and learning as a lifelong pursuit. PLNs can be powerful tools for extending learning, though I am not entirely convinced by some of the tools usually suggested as being essential to a PLN - for example, Facebook is a wonderful tool for social connections, but seems poorly designed for connecting learners.
Twitter is better. its brevity and focus on shared links makes it a perfect tool for connectivism - find a few users with shared interests, follow them and then follow their followers. You will soon have a timeline of tweets full of ideas and links to content that would have taken you ages to find by other means. Delicious has also improved no end recently - related links can now be shared using the concept of Stacks: lists presented in text, media or hybrid formats. Stacks can range from the very general to lists which focus on specific issues and developments. Finally, iPad users should seek out Zite as a starting point for new connections - its a personalised magazine with channels devoted to education and e-learning, amongst many others. As well as online newspaper articles, it links directly to some of the most useful educational blogs, including Steve Wheeler's always throughtful and provocative posts.