Daily and monthly active user counts are the best metric. With X being private and the company’s public statements being clearly false, we have to rely on sites that track site traffic but I think the picture is clearly that there has been migration away from X. The better question is whether it will continue and to what degree.
The people that don’t like Elon or what X looks like under him have left for the most part. Those who like him or what it looks like under him are happy to be there - but there’s also a portion of users that aren’t that interested one way or the other and just want the content. Whether they get turned off at X or find other reasons to leave is probably where the next phase of migration is. Will it happen to any significant degree or not?