The FAA has just reiterated its position about hooking up with paying passengers on the internet unless you hold a Part 135 operating certificate. Many of you followed the Flytenow path through the Federal courts, where the FAA's long-held position on the issue was affirmed. Another company recently tried a slightly different approach to that same idea, and the FAA jumped on them quickly. Anyone thinking about offering expense-shared rides via the internet or other public venues should review the following document which summarizes the FAA's position and provides links to the legal guidance on point.
BTW, notice in the letter to Blackbird's attorneys the FAA's intent to get hold of the names of the pilots picking up riders this way. I certainly wouldn't want my name on such a list, and I don't think there's any "expectation of privacy" when you make a public offer on the internet.