In the UK (I don't know about the US), definitely 'No'.
You're not allowed to advertise the flight publicly (except for in a flying club premises) - so no Social Media or web advertising (including forums) of spare seats if you want to share costs.
From the ANO regarding cost sharing of private flights:
(a) no more than four persons (including the pilot) are carried;
(b) the proportion which the contribution referred to in paragraph (2)(c) bears to the
direct costs is not more than the proportion which the number of persons carried
on the flight (excluding the pilot) bears to the number of persons carried (including
the pilot);
(c) no information has been published or advertised before the commencement of
the flight other than, in the case of an aircraft operated by a flying club, advertising
wholly within the premises of such a flying club in which case all the persons
carried on such a flight who are aged 18 years or over must be members of that
flying club;
Otherwise, it's a public transport flight.
Last edited by wb9999; 24th Mar 2014 at 17:34.