I was the PM for this at Twitter. You pretty much nailed it. The straight Address Book algorithms are Reverse Address Book (your #1) and Strong-ties prediction (your #2 and #3).

LinkedIn for sure also uses common groups (schools, workplaces, interests, etc), and geolocation in their recos. There's also sneakier signals like app graph and browsing history, though in my experience those were more useful for content recos than people recos.