There's a bit of a difference between the company owners starting conversations between themselves to create content interesting enough to attract outsiders and robots (indirectly) asking you to pay so you can reply to their messages.

The most underhand variation of all I've encountered was sms.ac (ironic acroynm) which suckered you in with Bebo-style friendspam to join their "free SMS" network, at which point your blank profile started receiving unsolicited reverse charge "flirts" from random non-humans.