What's the purpose of the favorite button?
You can find links to some basic info on favorites here, specifically this post is rather detailed: How do favorite questions work? This feature request has been posted on main meta a few years ago: ...
Is it reasonable to expect that the number of upvotes is greater than the number of favorites?
No, not entirely. An average question indeed has more upvotes than bookmarks (the new name of favorites). See below for some statistics. Both vote types (bookmarks are stored as votes in the database) ...
The meaning of "The number of Followers of a tag"
The number of followers is shown in the tooltip which is displayed when you hover over a tag. As already explained in comments, this is number of users who have the tag among their favorite tags plus ...
Any chance of a "save for later / TODO" feature?
The feature called "bookmarks" was recently completely overhauled by Stack Exchange and it is now called saves: Bookmarks have evolved into Saves Recent feature changes to Stack Exchange - ...
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