I agree with your drift, Whopity, but is there any case law on this?
IMHO, IANAL, etc, but I don't think the CAA is going to fall over itself to rush to Court to test this one, because there is a fair chance that a www-based club is a club like any other.
The world is changing all the time. I hate Fa**b00k and Tw1tter with a vengeance (kids would be so much better off making
real friends, and wasting less time and their parents' money on their phones) but grab 1000 teenagers and ask them if their Fa**b00k community is real or fake...
The "seat sharing" websites are obviously advertising cost sharing, primarily. Many (most?) rental-level PPLs will not do any flight unless they can cost-share, and the schools offer a very limited opportunity for digging out passengers. The cost share is agreed off the site, by email.
There has never
AFAIK been an attack by the CAA on seat sharing websites.
That said, the OP should be careful because under the UK Civil Aviation Act there is no passenger liability unless the pilot is found negligent, so any injured passengers have a huge financial incentive to lie about the preceeding financial arrangements; if they can show that the pilot did something illegal they will get a payout, and if they are not pilots themselves they have nothing (no license) to lose by perjury because they can always claim they are not aviation people and do not understand the ANO... So if I was cost sharing I would probably do it in a manner which leaves a record
on the ground of the agreed arrangements.
A lot of people cost share by doing it only after landing, but that doesn't solve the problem.
I am N-reg so cannot cost-share anyway, so I don't.
A flying "blog" itself is a non-issue; I have trip writeups
here. No invitation to cost share is implied.