Right now you need to do it with a plugin. In the Language-ext.properties file, add lines like:
user.profile.var1=Twitter ID
user.profile.var2=Facebook Page Link
I believe you can add up 25 fields like this.
There's another option that would take a little more work to implement, but you could create a structure to hold your extra data, and use the user account ID as the tie in. Ultimately, I think the goal is to have everything like user data become structured content, so you can extend it internally (like they did with hosts in 1.9 and files in 2.0).