Right so i've been using Mylikes for a couple of weeks now (This is my second attempt, the first time i was banned). But i have figured out the reason for what you are referring.
Originally i was using Tumblr to generate traffic and my click through link was the Mylikes bitly link i was given by the site. I at first would get a large amount of clicks e.g 1,000 and get like $5 from that but then later that evening my earnings were revalued to like $1.80 or something to that equivalent. In the end my account got banned and i was done with this anyway.
However a couple of weeks ago i decided to try Mylikes again on my facebook pages (I own a network of 10 pages totally around 300K). I'd already been prepping them for weeks with funny pictures but last week i started slipping in a couple of Mylikes links per day. The first post made like $5 but then later on i was astonished to found that when i logged in my earnings had been revalued and they had doubled instead.
So my conclusion on Mylikes is this. If you are using a poor traffic source like Tumblr where the majority of the clicks are accidental then people aren't going to actually be reading the content they are providing and therefore not seeing their advertisers adverts. As a result of this they will put your earnings down, especially if a lot of those clicks are coming from the same people. I imagine the same kind of thing can happen with twitter because you are always promoting to the same group of people.
Facebook seems slightly different because there is a slightly more viral aspect which means that you content can get spread far further and i believe this is happening to me. I'm now getting $0.025 for UK AUS & CAN clicks, whereas when you sign up you'd be lucky if its $0.008. So this just goes to show that quality trumps quantity and it can severely hurt your chances in the long run.
Lastly, you might consider doing a joint venture with someone on this site who's site you can send traffic to and then share in the ad revenue, may well have a better chance at getting money doing that.