Twitter is relevant because a very specific audience of multipliers (particularly journalists, but also other influential public figures) use Twitter - often in person, while some marketing company runs their Facebook account. The public reach that Twitter can potentially provide is much bigger than that of any other social network, even though those networks may nominally have a multiple of the number of users of Twitter.
Musk gets this, and it is why he is interested in Twitter. The market doesn't get it, which is why it undervalues Twitter. However, you cannot blame the market for that; it is poised to undervalue Twitter, because the actual value of Twitter is not accessible to just anybody, but people like Musk, who are well-connected in the Twitter social graph. Hence the value of Twitter to someone like Musk is actually much larger than the value of the exact same Twitter to any random shareholder.
