The "monetize" level

Some thinking going on lately about when it is time to think of what you do online as something that has value for others and can create value for yourself. What got me thinking about this was that one good friend wrote a congratulatory article about a pizza place and I got a question (probably a joke, but got me thinking) on Twitter about some congratulatory remarks I made. In both cases the underlying assumptions were that we, my friend and myself, were not doing that REALLY because we thought it was true (the pizza was really good and the tweet thing really interesting) but because we had something to gain: money, favours etc.
So the questions arose: when exactly does what you do online begin to gain some measurable value? does 500 followers on Twitter make me a valuable resource for messages? Also, why, after a certain limit, any personal opinion becomes challenged as "paid for" or "biased by interest".
What interests me here is the measurable level at which your opinion stops being taken at face value and become a political gesture (political meaning carrying an agenda).
.... hmmm, food for thought