From what I understand, they're likely trying to kill the fediverse by making it irrelevant (embrace, extend, extinguish) seeing how it's finally starting to grow, since they can't just buy it up this time like they've always done to competitors.
Even aside from that though, their algorithms designed to retain user attention by any means necessary are definitely going to seep into and poison the fediverse, at least indirectly, if they're federated.
Not to mention they could easily run ads as normal posts and boost them artificially; they are an ad company after all. Wouldn't put it past them.
Not federating with them means we don't have to deal with all that, and the fediverse can just continue to grow naturally as it's been doing.
Federating on the other hand means a very real risk of permanently halting the fediverse's growth in favour of corporations', like Google did to XMPP