The library of congress has been generating a 300 Terabyte database of 500 million daily public twitter posts dating 2006-2010.
The issue is size & relevance: Only 13% of Americans have Twitter accounts compared with Facebook, at 70%. The majority data returned by twitter are bits and pieces of words, butchered, with no relevance or substance.
I do not think we should archive facebook either, but the comparison is that facebook is a diverse, community, more reflective of American Life, with the ability of users to PUBLISH Stories, Poems, Songs, News, Music, and Photos to say the least. That is the kind of Content we want to pass to our children; Not "wat u doin" from 50 million people.