Of course Twitter is doing nothing uniquely unusual in this respect, as these are just 7 examples from a pool of some 300 announcements of unallocated address space (a list of such bogons can be found at http://www.cidr-report.org/as2.0/#Bogons)
:)
- Why is Twitter announcing these prefixes?
I have no idea. Something has gone wrong here and the address has come back to the RIR and Twitter apper to be unaware of this.
No, Twitter is absolutely aware of this issue, I alerted their NOC when I got the result this morning from CIDR report (yes, I scrop your data daily) but unfortunately there response was "This prefix is valid and owned by us in RIPE region. Please do your homework before making incorrect accusations." But atleast I tried.
- How and why is this prefix in RADB, given that it is unallocated space?
Good question - I wonder what periodic checks the RADB undertakes on the data held in its registry?
No idea, it should be triggered right away when the RIR, who is the authentic source of these resources marked them "Unalloacted". But in a perfect world.
- Why do upstream AS’s accept these advertised prefixes?
Maybe they chose to believe that RADB performs robust periodic integrity checks? Or <insert reason here>?
Yes, mostly follow RADB.