Key elements are a facebook blue colored hat & facebook blue hued shirt paired with a white sleeveless shirt. I bought the following items new with the intent to reuse them afterwards, but you may find something in your/their closets that will suffice:
Old Navy Performance Fleece Hat in "Blue Eye" priced at $7.50.
Target Boys' C9 by Champion® Blue Long-Sleeve Compression Shirt priced at $9.99.
Target C9 by Champion® Power Workout Tee in white priced at $7.99.
Other supplies needed are: Clear packing Tape, scissors, & a printer with paper
I began by printing out an enlarged version of the facebook logo and carefully cutting it out. I then taped it to the fleece old navy hat with a single strip of the clear masking tape.
Next, I printed out my son's current "wall" on facebook with a newly typed status saying he was off to a Halloween party. I taped page 1 of 2 to the front of his white shirt. Covering the whole sheet from weather and spills with strips of the clear tape. I then taped the other on the back, fully covering it for protection as well. (Another idea would be to use iron-ons of the facebook pages, but I liked the concept of being able to use the shirt over again without the design.)
Pants can be anything, my son plans on wearing grey track pants with a blue stripe from his closet.
Older twins, couples? Make a twitter costume too! Follow the same basic concept & instructions adjusting the coloring to twitter blue. Old Navy even has a women's white performance fleece hat that would make a great base for the blue twitter font logo.