The simple answer would be Facebook Events, but truth is that there are many other ways to promote your online event on Facebook. When promoting an online event on Facebook you want to use the full set of tools that Facebook provides to make sure that people know about your online event and show up to it.
Tell your inner circle
The first thing you can do is post about your event on your wall and story. You can post something simple such as a poster for the event or a picture from a previous event. You can add a short description but remember to keep it short as the optimal length for a Facebook post is about 40 characters (not words!). If you can avoid it it's best not to include a link as Facebook punishes for that in its algorithm. Instead, you can encourage people to engage with the post such as saying "DM me" (Direct Message) or "Comment Below". Then you can send a personal invitation to people who show interest with more information. You can also try tagging people in your posts such as other event participants, organizers, or just friends you think will be interested.
Share to a Group
The next circle you can share with is people who are in groups that share an interest in the online event you are holding. For example, if you are hosting an online language exchange for English and Spanish you can look for a group "English Spanish language exchange", "Learn Spanish", "Spanish speakers in London" or even search in Spanish for some groups. Once you join these groups, you should check what rules they have regarding promotional posts. Try and adjust the nature of your post to what the group allows. You can write a new post for each group (best) or share the post from your own wall. Try and change the wording of the post between groups and space them out over time because Facebook can pick up multiple similar posts which it punishes for (because they want you to buy ads from them). Make sure to keep track of all your posts and engage with people that show interest.
Join the conversation
Sharing your own posts is not the only way to promote your online event on Facebook. You can also join the discussions in various groups and comment on other people's posts that show they might be interested in your event. If we continue with the "Spanish English language exchange" example then you can join a group for finding language exchange partners and use the search function to find posts about Spanish or English. Then you can comment on these posts with helpful advice or an invitation to your event. It's important to only comment on posts that seem genuinely interested in what you have to offer otherwise you can be tagged as spam. Nonetheless, comments are a great way to promote your event in groups that won't let you post since group admins have much less control over them.
Create buzz
You don't have to post explicitly about your event to promote it. You can create buzz and conversation about a related topic in order to identify people who might be interested in your online event. If we continue with our "Spanish English language exchange" example then instead of posting "Please come to my event at so and so time follow this link:". You can post in a group "Who is learning Spanish?", "What is your favorite Spanish TV show?", "What level is your Spanish?" etc. Facebook is usually "nicer" to simple nonpromotional content, and other people are more likely to engage with it.
Create a Facebook Event
It's not the only way to promote your online event on Facebook, but it also shouldn't be ignored. You can create an event as an individual or you can do it from within a group you manage. When you create the event make sure to choose the "online event" option, and supply all the information necessary to connect to your meeting. If your online event is a weekly event then you can just duplicate the previous week's event and adjust any details that need to be fixed. Creating an event on Facebook has several advantages. You can invite friends you think will be interested, you can track the people who respond to your event and show interest, and also Facebook will show your event to people it thinks will be interested.