Ah – something which I’d done prior to 23 Things starting is setting myself up on LinkedIn. It’s a useful way of staying in touch with colleagues, groups (some organisations are closed groups i.e. you must be a member of the group to join its LinkedIn presence), companies and other organisations. A LinkedIn profile can have many sections – but it’s easy to tailor. You can also add details of certificates gained etc. Take care when deciding who can edit your profile!! Read up about 1st, 2nd etc. degree contacts.
I enjoy getting news when someone changes jobs or has a work anniversary – and also recommend people and endorse their skills / expertise when appropriate.
LinkedIn isn’t a place for listing papers, so that’s where Academia.edu and ResearchGate come in. Having Googled colleagues, most of mine use ResearchGate so this is the way that I’ll go. It’s great that you can get papers from colleagues and make new connections. I’m intrigued by “Out of Curiosity’s” comment about bookmarks in Academia.edu but not in ResearchGate.