T-Mobile branded prepaid subscriptions grew by 321,000 in 2011 with most of the growth (220,000) occuring in the fourth quarter. I think credit for that growth should go to the $30/month 100 minutes, unlimited messaging and 5 GB data plan that was introduced on October 16th.
T-Mobile also gained 780,000 prepaid MVNO (mainly América Móvil and SIMple Mobile) users in 2011. At year end T-Mobile had 4.8 million branded and 3.6 million prepaid MVNO customers for a total of 8.4 million total prepaid customers. That makes T-Mobile the 4rd largest US prepaid operator after América Móvil, Sprint and MetroPCS.
Unfortunately, the prepaid additions were not enough to support massive subscriber loses on the post paid side. T-Mobile shed 1.6 million contract customers in 2011, 800,000 of them in Q4. T-Mobile blames the loses primarily on the fact that it is the only major US operator without the iPhone. However, I think uncertainty over the aborted AT&T merger, declines in customer support satisfaction and a lackluster lineup of high end contract phones also contributed. T-Mobile finished the year with 33.2 million total customers, making it the fourth largest US operator after Verizon, AT&T and Sprint.
On the positive side T-Mobile's profits and average revenue per user both increased in 2011. T-Mobile USA CEO Philipp Humm promised that "T-Mobile USA will invest to get the business back to growth, including an incremental $1.4 billion investment in its network modernization initiative, which will total a $4 billion investment over time.” The network improvements will include T-Mobile finally launching an LTE 4G network in the US beginning in 2013.