I think your question is phrased in a somewhat misleading way. For starters, I gave 10% as a loose lower bound, not as the actual number. More importantly, the question implies that a user is active if and only if he/she syncs a file today.
There are a variety of things that can be used to define "active user". Logically, the point of defining "active user" is to differentiate between users who have a good chance of using a product/service again or on a regular basis or something like that (depends on the nature of your service), and users who almost certainly won't and are just taking up space in your user registry (or in our case, space on our servers as well). I won't go into any further details, but the number of people who do a file sync at least once a week is much higher than the number of people who do a file sync every day, and the number of people who do a file sync at least once a month is much higher than that. In addition, there are people who don't add or edit new files very often, but do frequently access their existing files, who aren't being counted in the approximate numbers I gave above. Anyway, going off of this qualitative "definition" of activity, I consider much, much more than 10% of our users to be active users.
Also, well over 100K users actively sync a file each and every day.