Over time people from that initial spike realized that the platform isn't what they expected/liked, some likely went back to their previous platform but i am going to be optimistic and say they moved to mastodon.
The climb we see now is fresh users that heard of lemmy and are now checking it out.
//This is how i read this graph, other interpretation may be possible, you can't really know for certain without more data.
Traffic peaked at the reddit migration, after that, people either left or became mostly lurkers, and since lemmy versions before 0.19 didn't count votes as activity most users were not included in the graph.
Now that instances are upgrading to 0.19 there's an increase of activity.
0.19 broke federation for quite a lot of people for the majority of December, probably has something to do with it.
I've been an almost daily Lemming since last summer but when nothing I posted was making it off my server for about three weeks there wasn't much point!