Currently this is not possible.

How do they link their account to twitter then? I saw the settings in the backend but I do not see them in the front end.

This is only for auto posting You can do so when they edit their profile on the front end

I guess I can use the link custom field for this.

What does the skype custom field do?

Hi there,

Skype custom field are used to display a nice click-to-call button on user's details as you can see from my screenshot here, http://screencast.com/t/zzOMSyWjcvZk . Once other user click on the button, it will automatically launch a skype applications on your PC and you will be able to call or chat with that person.