Isn't it pretty much expected? There was a massive boost during the API controversy then it dropped off as the number of active users stabilised. I don't think Lemmy will immediately replace Reddit like people want it to, just a slow gradual migration of communities. After all the migration from Digg to Reddit took years. I've got no doubt that Reddit will make more dumb decisions that will push more over to Lemmy.
Honestly I just want communities like /r/framework to move over and some UK subs like /r/frugaluk and /r/casualuk then I'm happy.