Agreed that this is an issue, but it's a smaller one than you make out: Consider that there's likely a marked hockey stick going on here. Now keep the very most recent tweets of the very "worst" in terms of followers in a hot cache, and mix their most recent tweets into the timelines of their followers as needed until they're committed everywhere. Then collapse writes, and you reduce the "conversation" problem.
It's still not cheap to handle, but consider that fan-out on write is essentially an optimization. You don't need that optimization to be "pure" in that you don't need to depend on the writes being completed in a timely fashion - there are any numbers of tradeoffs you can make between read and write performance.