Erm, 3000/month is more than possible
My current mobile plan offers me 3000 SMS per month...i usually breach this limit
Most people I know also go way over the 3000 inclusive SMS (Sending only, no limit for receiving)
Includint the fact that people also SMS to Twitter/Facebook and such, these numbers are entirely possible, as every 5 minutes i see a "Facebook for Samsung" or "Facebook for iPhone" on my FB Status updates alongside the normal "Sent via Text Message" ones. In fact, I would say only 10% of the status updates i receive are from the website itself.
When you include the fact people also receive SMS's when a new status or inbox message on Facebook arrives (unsure about twitter or other services as i don't use them), you can be looking at around 300-400 inbound from facebook ALONE per day, and even replying something as simple as "lol" would count as a sent SMS. So say you receive 400 per day from facebook, and 50 from friends, you reply to all your friends messages (send 50) and 1/2 the facebook ones (200), thats 250 per day, multiply this by 1 month, and you have sent 7000 messages and received 12600 messages, making a grand total of 19600 per month. Entierly possible for people with a Facebook Friends count of around 800 people (Most teenagers...as they add random people, and people they have never met).
I am only using these figures as an example, as they would obviously go up during peek hours and days (Weekdays, Friday Nights and Saturday nights), whereas Sunday is mostly slow, but still has lots including "Urgh, so hungover, anyone else feel like hell?"...to which, you would reply...causing a spike in numbers even on slow days.
Forgot to mention tha quite a lot of people are now using blackberry chat, and also MSN Mobile, which, although its not technically 'Texting' in its literal sence, it is still using your mobile, for written communication. So each sent and received message would also count (Concidering the average conversation is approx 200 lines of text (100 per user in a 2 person conversation) - If your in multiple conversations, multiply this by however many they are in (Teenagers would likely be in multiple chats with classmates and friends) - So yeah...3000...highly possible, if not a much lower figure than the study calculated.
Could you class MSN/IRC conversations on the computer as 'Texting' as you are still technically sending and receiving written messages...in which case...im possibly on about 8000 sent messages today and 12000 received....already 17000 more (I reside in around 8 different IRC chat rooms, and my MSN is my primary source of communication with clients and friends to lower the cost of phone bills) than the average 'monthly' texting figures