Doesn't seem surprising. It's the new shiny thing and then interest wanes a bit. Probably same holds true of Lemmy IMO. That said Threads is now a viable replacement for Twitter and advertisers might decide one platform is a toxic cesspit and the other isn't (and has some big crossover influencers) and spend accordingly. Even if it only hurts Twitter by a few % of revenue, that still more losses for Musk's ego purchase and it should be seen as a good thing.
Threads is allegedly going to support ActivityPub so theoretically itself and Mastodon, and Lemmy could all have some kind of federated access to each other. But Threads is an enormous whale in a pond of minows so how that would work is anyone's guess.
Social networks that end up being successful have a long initial growth phase. Interest waning after a couple of days is a terrible sign. Threads is already dead.
Way too early to proclaim that. You might as well proclaim Lemmy the same - after all, the controversy over Reddit will fade in time and so too will interest in moving away from it.
Lemmy is a completely different thing. It is a grassroots movement of geeks motivated by an ideal. It has some chance of having enough people "fueling the fire" while it grows slowly and, also importantly, it does not need to grow like crazy to survive. This contrasts with threads, which is an attempt by a big tech company to capture mainstream audience. The mainstream has the attention span of 5 seconds, forms an opinion and never goes back. Either it is immediately fun or it is a flop. Threads had the best possible shot, riding on the enormous unpopularity of Musk and being able to piggy back on the huge user base of Instagram. They took the shot, it didn't work. The only way they can get a second shot is if another extraordinary event happens. I will admit that given the current tech leadership (and specifically Musk) this is not impossible, but otherwise I'm pretty confident in declaring Threads' stillbirth.