We update your fan counts once every 24 hours. We pull the numbers from your social media accounts once per night, so any new followers/likes/subscribers you gain on your socially synced sites should be updated overnight.
If your fan counts are still incorrect, first try disconnecting your social syncs and then re-syncing the accounts. That will hopefully correct the issue during our next nightly update.
Please note that it is not uncommon for syncs to require ReverbNation to request/receive information from the synced site in question. This means that sometimes issues with Facebook, Twitter, or YouTube may have to be resolved on their end.