I just made a chatter post with the #Awesome hashtag.
When I queried for this tag, it didn't appear straightaway.
When I tried again about 15 minutes later, the query returned the tag.
Whilst waiting I also clicked on the tag once to find all posts with that Hashtag.
So I would think there is an indexing job that runs periodically and scans posts to create HashtagDefinition records, rather than becoming available realtime.
This query result kinds ties in with the theory, tags I created at 20.03 and 20.05 only appeared at 21.00.31 - notice how the SystemModstamp on all 3 are the same. Signs of a batch-like process. (The Count increased for 1, and 2&3 were created)
CreatedDate Name SystemModstamp
1 2012-11-03T19:42:38.000Z Awesome 2012-11-03T21:00:31.000Z
2 2012-11-03T20:03:50.000Z wonderful 2012-11-03T21:00:31.000Z
3 2012-11-03T20:05:48.000Z Love 2012-11-03T21:00:31.000Z