The availability and accessibility of psychological support is critical for the global population's mental health. Artificial social agents like chatbots have recently gained popularity, showing a growing potential for their use in mental health care. The current study investigates how users express emotions online when interacting with a commercially available chatbot (Simsimi) and posting on a public social media platform (Twitter). Our results suggest that private conversations with a chatbot tend to be more emotionally charged and focus on personal issues more frequently than messages posted on Twitter. Our findings suggest that private conversations with a chatbot are more emotionally charged and more likely to focus on personal issues than messages posted on Twitter. Users may feel more comfortable sharing mental health issues with a chatbot than on social media, during major upheavals like the COVID-19 pandemic. Our findings show the potential for personalized chatbot-assisted mental health support, particularly in the event of long-term disasters.