Mellowads/Zap.in paid me around 20 satoshis for 3,615 link clicks. Each click was validated too because it was generated by their API and paid out via a faucet. All those clicks for only 2 cents? That's absurd...I messaged their mellowads email, zap.in email, and multiple facebook pages more than once to get an answer but I never heard a thing back from anywhere. They seem to work until you need something sorted out because they have absolutely no support. It stinks too because they are such a huge network.
at the end of the day mellow is just an ad network that works as a free market.
meaning that advertisers are basically paying roughly what they think your traffic is worth - if you aren't earning much then that's because advertisers dont value your traffic much (i realise that can be hard to accept for some people sometimes!).
mellow don't fix or manipulate this in any way, other than taking their 10% fee of the daily network pool.
what they do have control over is attracting new advertisers etc - and i am sure they work on this constantly as its in their interest too (their 10% of bigger advertising spend will increase)
i imagine earnings might be down slightly at the moment due to a number of external factors...
1. bitcoin network congestion - slow/expensive for advertisers to deposit funds quickly
2. holiday season in many parts of the world - advertisers may be taking a break
3. bitcoin price fluctuations - earnings in btc are much lower but this is still a high USD amount
in my experience they are still the best paying ad network (probably mainly due to having the lowest fees) - and most reliable (you get paid daily by bitcoin)