Requirements Engineering (RE) and all its underlying activities, such as requirements identification, evolution, validation, communication, and management, are still the key factors in successful product development. Therefore, proper implementation of this process is necessary to obtain a quality product. A better understanding of the most challenging RE-related topics for practitioners will greatly help to identify the areas of RE that may require extra attention by researchers and project managers. However, there has been very little experimental work towards identifying a practitioner's needs on the implementation and understanding of RE activities and tasks. Therefore, in this paper, we use data from popular social Q&A sites (i.e. Stack Overflow, Programmers Stack Exchange, Project Management Stack Exchange, and Quora), and analyze 4,553 questions and answers to examine what requirements engineers' needs are and what they ask about. To this end, we applied Latent Dirichlet Allocation-based (LDA) topic models and statistical analysis to explore the main related topics to RE. Our findings show that software practitioners are asking about requirements communication, evolution, validation, and modelling. Furthermore, we determined common RE challenges and issues, identified the main types of questions practitioners ask (i.e. what, how, why, when, where, and RE domain), and categorized these questions based on various aspects of software products (e.g. functionality, quality, and release planning). Our findings help highlight the challenges facing requirements engineers that require more attention from the software engineering - specifically requirements and product engineering - research communities in the future and establish a novel approach for analyzing the content of social Q&A websites.