[extesy]

I don't know their implementation but I can think of two workarounds for the firehose issue:

1. Paid approach: use some authorized firehose data resellers. That's expensive, but if there are already thousands of users, that will pay off.

2. Free approach: use poll-based solution to search for the keyword every X seconds/minutes. When new tweet appears in the search results after the last recorded tweet - that's your new tweet. This way it shouldn't miss anything and you don't need firehose. Of course, to overcome request throttling limits, you'll need to use multiple different credentials.