Just some general thoughts on this FAQ.
It doesn't specify the max field length (40 characters as per data limits FAQ) which I think might be worth including?
Second paragraph refers to using same keyword for multiple pictures e.g. if "foo" exists then "foo, 1" is rejected while "foo: 1" say, is ok (by virtue of comma making 'foo' in "foo, 1" a keyword in own right).
I'm pondering on whether the second paragraph is a little unclear. New users may just look and think "friends at mall" is three words not one, which would be confusing? Or, they may end up wondering whether or not there are other benefits to multiple keywords separated by commas which make a difference such as searching by keyword!
I can't think of better wording for the paragraph right now myself though. :-/
Also, details of what non-alphanumeric characters are permissible could be good - needn't be an exhaustive list; just some examples. Could help users frame ideas for categorising them to make them easier to select or look through (esp. with +15/50).