FacebookQuestion.java

import java.util.ArrayList;
import java.util.HashMap;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.Scanner;


public class FacebookQuestion {
	
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		Map<String, Node> allNodes = new HashMap<String, Node>();
		
		int numPairs = sc.nextInt();
		for(int i = 0; i < numPairs; ++i) {
			String left = sc.next();
			String right = sc.next();
			
			Node parent = null;
			Node dep = null;

			if ((allNodes.containsKey(left)))
				parent =allNodes.get(left);
			else 
				parent = new Node(left);
			
			if ((allNodes.containsKey(right)))
				dep =allNodes.get(right);
			else 
				dep = new Node(right);
			
			dep.isHead = false;
			parent.addDependency(dep);
			allNodes.put(left, parent);
			allNodes.put(right, dep);
		}
		
		List<LinkedList<Node>> list = new ArrayList<LinkedList<Node>>();
		for(Node n: allNodes.values()) {
			LinkedList<Node> nodeList = new LinkedList<Node>();
			if (n.isHead) {
				Node crawl = n;
				while(crawl.dependency != null) {
						nodeList.add(crawl);
						crawl = crawl.dependency;
				}
				nodeList.add(crawl);
				list.add(nodeList);
			}
		}
		System.out.println(list.toString());
	}
}

class Node {
	Node dependency;
	String name;
	boolean isHead = true;
	
	public Node(String name) {
		this.name = name;
		dependency = null;
	}
	
	public void addDependency(Node dep) {
		this.dependency = dep;
	}
	
	@Override
	public boolean equals(Object other) {
		Node otherNode = (Node)other;
		return otherNode.name.equals(this.name);
	}
	
	@Override
	public String toString() {
		return this.name;
	}
}

Nice. Much much much cleaner than my solution: https://gist.github.com/dejesus2010/5ff415f99898c378ee87

Here's Will's solution: http://pastebin.com/AdEJuyNQ