It's impossible to know ahead of time which tweets or trends are going to be interesting.

True, but filling the records with so much noise that finding the interesting ones amounts to concentrating gold from seawater, you’re no longer doing the job an archivist should.

If you have the firehose, it is easy to derive the selective archive by filtering on the list of noteworthy accounts. Ignoring monetary costs, archiving the firehose is strictly better than selective archival. It enables future researchers to examine trends on a societal scale.

It does, at the cost of preserving the firehose, through endless format and media shifts, and at the cost and under the assumption that future historians will want an archive they need a strong AI to manage. There is also the cost, as in paying people and buying equipment, drawn from an already overextended budget.

Frankly, the firehose will be considered priceless in 100 years. The accounts and tweets that we care about now will also probably be the least valuable. Historians and archaeologists will generally take one grocery list over a thousand sweet nothings from politicians and celebrities.

Preserving minutiae for future archaeologists is a bold and exciting idea which should be funded and pursued. It is not however, the mission of the LoC.

Edit: not to mention that I think you have a far too rosy view of the world we’ll be leaving for people 100 years from now.

Agreed. Hell, I've recovered old hard drive backups from ~10 years ago and have been fascinated by by what I've found. Old e-mails and chat logs and stuff can be very entertaining to dig through.

Or find things that seemed insignificant at the time but became significant later with hindsight.

not to mention the fact that twitter is basically millions of monkeys banging away at keyboards, eventually they will produce some great works.

For values of “eventually” which include timescales on the order of proton decay...

...like a particular painter and WW1 veteran in Germany that the than current art world dismissed...

There's so much valuable research that can be done with aggregate Tweets that you can't do with just lone accounts. For example I've seen flu outbreaks being predicted by hashtags. (http://money.cnn.com/2017/05/11/technology/tracking-flu-twit...) The possibilities are really endless, but depend on whether you have the data and whether it's over a large enough timescale for your use. Twitter represents a large portion of our culture and just because you don't personally like it or have a use for it, doesn't mean we should just throw away archives of it.