FacebookDecoder.java

public class FacebookDecoder {

	public static void main(String[] args) {
		test("");
		test("0");
		test("01");
		test("1001");
		test("12");
		test("26");
		test("27");
		test("101");
		test("1010");
		test("10210");
		test("262");
		test("272");
		test("362");
		test("1234567890");
	}
	
	static void test(String s) {
		System.out.println("\"" + s + "\" -> " + numDecodePossibilities(s));
	}
	
	static int numDecodePossibilities(String s) {
		int count = 0;
		if(s.length()==0)
			count = 1; // empty String translates to empty message
		else {
			int current, prev = 0;
			for(int i=0; i<s.length(); i++) {
				current = Integer.valueOf(String.valueOf(s.charAt(i)));
				if(prev != 0) {
					if(prev*10 + current <= 26)
						count++;
				} else if(current == 0)
					return 0; // 2x consecutive '0' is not allowed, covers also strings starting with '0'
				
				if(current != 0)
					count++;
	
				prev = current;
			}
		}
		return count;
	}
	
}

Output:

"" -> 1
"0" -> 0
"01" -> 0
"1001" -> 0
"12" -> 3
"26" -> 3
"27" -> 2
"101" -> 3
"1010" -> 4
"10210" -> 6
"262" -> 4
"272" -> 3
"362" -> 3
"1234567890" -> 11