This is a question that a lot of peoples are asking for these days of
Januar 2018. I have seen that a lot of people are terrified, about this. I am using for some years adsense, and this is completely normal for every beginning of every year, so don't worry about this big drop in your earnings on ads or other platforms.
I am going to mention some reasons :
1 - Christmas season is closed - A lot of advertisers make ads in the end of the year by making offers for they products, and as you know, most of the peoples buy things in the end of the year, like clothes, mobile phones, tv etc. So these ads from companies are ended now. These companies give a huge money for advertisements. And now are ended all the campaigns.
2- End of the budget for a lot of ads.
Some ads have a yearly budget and they have to use for all year, and when the end of the year comes they have to spend it all, in case to close and to be ready for the next year, and this is another reason why earnings are higher in December than other months.
Don't worry about low earnings on first months of the year Januar, February, after this month's earnings will grow slowly day by day. All you have to do is just keep working every day regularly