Sorting Arraylist By Quantity

I'm getting this error:

The operator < is undefined for the argument type(s) java.util.ArrayList<FacebookUser>, java.util.ArrayList<FacebookUser>

For this:

class FriendsComparator implements Comparator<FacebookUser>
{
@Override
public int compare(FacebookUser o1, FacebookUser o2)
{
int returnValue = 0;
if (o1.friends < o2.friends)
returnValue = -1;
[Code] ....

Same as for the second if condition. How can I fix this exactly? What I'm doing is trying to sort Facebook users by the most to least amount of friends.

Here's the Driver:

import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectOutputStream;
import java.util.Scanner;
import java.io.BufferedInputStream;
import java.io.BufferedOutputStream;

[Code] .....