Facebook said growth in the number of users using Facebook on mobile devices, which is hard to monetize, "may negatively affect our revenue and financial results."
In an amended regulatory filing, the company said the number of people logging into Facebook is continuing to grow more quickly than the number of ads delivered.
Facebook said this is in part because more people are using the social network on mobile devices, where it shows a very small number of ads.
This implies that Facebook has room to grow in the still-nascent mobile advertising space.
Facebook said it saw that trend in the first quarter, and that has continued in the current quarter.
The changes came after executives met with prospective investors in New York and Boston ahead of its initial public offering of stock. Facebook added three paragraphs related to mobile usage to its filing. The additions were likely in response to questions from investors.