Yes, same goes for me too. I normally get a grav drop soon after logging in and then nothing more. I have never had more than one per day.
I read earlier in this post that someone knew of players that have "emptied their wallets" to get gravitons, 100 in fact. Well I must ask, if this is true then....... why?
100 gravitons at the discounted price of 590 gold = 59,000 gold.
100 gravitons at normal price of 1500 gold = 150,000 gold.
Gravs are used for (1)shielding of the Junkship (thus they are wasted in this manner due to 3:1 conversion AND the simple fact that they literally disappear. Charging up one shield costs 35,400 gold), and (2) for fun really in their 30 seconds of use!
Now I don't know about your currency values but in mine 50K gold works out to be about $75! With this I can get a few months supply of unlimited E and bp booster and speed boost! Much better value than something that "wastes away"!
Please be sensible in how you spend your money or you may regret it later once you realise there is nothing left to show for it.