All of the major wireless carriers have now reported their first quarter earnings, and it was an extremely strong month for prepaid wireless services. MetroPCS had a record 684,000 net subscriber additions, and Boost Mobile, Sprint Nextel's prepaid unit, gave Sprint 764,000 prepaid iDEN subscribers, the one bright spot in an otherwise weak quarter for Sprint. But after all of the calculating of the additions, which carriers have larger subscriber bases than the others? FierceWireless compiled a list of the top seven carriers based on subscriber base, using research from the firm Strategy Analytics. The list does not include resellers or MVNOs, such as Virgin Mobile USA. Smaller carriers, such as Cincinnati Bell, also are not included. IPCS reports its results Monday, but ended 2008 with 691,000 customers and therefore probably would not rank among the top seven. Slideshow