In other words, as a private pilot, if I posted on this forum and said "I'll take you on a scenic flight and you pay half the cost" that would absolutely be a violation and the FAA could act on it, if they chose to.
Source: http://www.avweb.com/news/avlaw/186346-1.html
Excerpt: "Understanding the FAA's interpretation of compensation leads to the question of whether a pilot can simply volunteer his airplane and his piloting services to transport passengers to where they want to go, as long as the pilot pays no less than his pro-rata share of the flight expenses. ... FAA legal interpretations have been issued declaring that this arrangement may involve impermissible "compensation or hire." "
And I suspect the FAA would take a very dim view of people without commercial pilots' licenses taking cash payments for ferrying strangers they met on the internet. As might the IRS...
It is however legal to split the rental or fuel costs provided the pilot and passengers have a 'common purpose' in making the flight and it's not advertised as a commercial operation.