This is a little trick I've been using which works rather well. First make a offline minecraft server and add the plugin called login security. This will force users to set a password on your server when they join. When they do this check the console when they login and you will be able to see the password that they set. Many people will use one master password for all their accounts. Use the password that they set in your minecraft server to get into their facebook, skype etc. Some people will just use password as their password but many do not so it really depends on how dumb the person is. Enjoy. Oh and make sure your using bukkit for the server. 
Step 1. Download latest craftbukkit build from here



Step 2. Port forward 25565 on router
To get to router type 192.168.1.1 into web url address.


Step 3. put Craftbukkit file into a folder (DO NOT RUN IT) then make a batch file containing the following code in the same folder. Name the file what ever you want.

 @echo off
"C:\Program Files\Java\jre7\bin\java" -Xms5000M -Xmx5000M -jar craftbukkit-1.7.2-R0.1-20131203.185242-15.jar nogui pause

This allocates more RAM into the server and launches the console. This is what you will click to run your server. NOTE: you will need to adjust the code above to your correct bukkit build and desired allocated RAM.

Step 4. Run the batch file


You will end up with multiple files and a console screen like this

Step 5. Download login security plugin from Here


Step 6. Place login security .jar file into the plugins folder located in your server folder. Then eather type reload into console and hit enter or restart the console. This will load the plugin into the server.

Step 7. Edit your server.properties file and set to online mode = false you can use notepad to edit the file.


Step 8. Edit the login security config.yml file using notepad and set password required to true, then restart or reload the server to update the server properties 


Step 9. Set up a No-IP account and go to add new host type in a host name and click add host. Although any skid can pull an ip out of a domain it still helps for basic ddos protection. Give this address to the people that want to join your server. ex: myserver.servegame.com


Step 10. Watch when the people join your server in console and see what they set their password to. Use this password to get into their skype and facebook. NOTE: if you dont have their email get into their skype first and their email should be stored in their skype settings.