Ping, WhatsApp, PMessenger and now MessageNow...boy they are crankin' them out now.
still, the only one that doesn't require you to 'register' is WhatsApp. and unless your friends are willing to constantly download another messaging app, (which mine are not) then i'd guess you need to pick one that they all use. since WhatsApp is still the only one that uses only a phone number (which my friends already have) that's the one i use. if i want to branch out and meet new people, i guess i could use Ping, PMessenger or MessageNow.
not knocking this app or any of the others, it's just that at some point i personally don't want 4 different messaging apps with 4 different methods of login (Pin#, username, screen name, phone#, etc). combine that with my IM, email, facebook & twitter credentials, and it just gets to be too much.
in all honesty to the creators of MessageNow, i wish you good luck and much success with your app!!