I can't find an exact breakdown, but the sources I can find say that ads are still a majority of Reddit's revenue. Reddit also has some structural advantages.

I can't find an exact breakdown, but the sources I can find say that ads are still a majority of Reddit's revenue. Reddit also has some structural advantages. Building around communities allows Reddit to ban advertiser unfriendly communities, and to threaten to ban others to force them to toe the line. It also gives them rough controls over what other users see (e.g. hiding NSFW content, quarantining subreddits so they don't show up on the not-logged-in front page). They've also managed to convince their users to moderate each other for free, which gives them a really lean head count. I think Twitter would need to substantially redefine their product to compete in that arena. Reddit also only made ~$350M in revenue last year. I don't think competing with Reddit is likely to justify the purchase price of Twitter over the timespan either is likely to survive.