$25M/year = approximately $2M/month = $200K/3 / day (1 month = 30 days)
At 3M tweets/day,
$200K/3 ~ 3M tweets
1 tweet = 2.222... cents
So, would you pay 2 cents for every tweet you send? Alternatively, if you were an advertiser, or a sponsor, would you pay the 2 cents required by your "followees" to keep their tweets flowing?
Take the poll on the right.
Remarks: Obviously, if just the number of tweets/day continue to grow, the cost per tweet will decrease. However, I'm assuming that Twitter's operating cost will increase with the number of tweets/day. Whether it's going to be a 1:1 ratio, that's up for debate.