Private messages are a means to vet those you are bantering with on the main forums. Not convinced MDN isn't a 16 year old car thief on the welfare computer at Gravesend Youth prison? PM him and ask him what colour the boathouse is at Hereford. Think that Chickenpunk may really be a 65 year old woman? PM her and offer to sell her a stana stairlift for 40 quid, see if the old bint doesn't take you up on the offer.
Sometimes, if you are lucky, you may get sex due to persistent PM'ing. That's what 90% of all PM activity consists of: forlorn efforts made by 2 woefully ugly people to convince each other that they're gorgeous and that really, they have a life - it's just that they've been made to 'stag on' this saturday night otherwise they'd be out on the pull, honest. DO NOT PM anyone who regularly posts on The Other Half or Lonely Hearts forums - they're proper gompers.
PM the ones you think may be girls, see if you can solicit a photo, I dare you. If you get any pics remember 'two's up'