What do tiny spiders feed on?

The past few days I have been accompanied on my commute to work by a spider the size of a nail head. It is suspended from the windshield by a thread. I do not know what it is up to. It has not started spinning a web, though it is rather agile going up and down on the thread.

It got me to wondering what spiders this size and smaller feed on. Do they spin webs that can catch much larger insects? This spider would be able to dine on a housefly for a considerable amount of time.

I don’t imagine it is going to find anything in my car. I should probably repatriate it to the great outdoors, taking care not to squish it in the process.