According to the white papers:

The Steem network is built upon Graphene, the same technology that powers BitShares. Graphene has been publicly demonstrated sustaining over 1000 transactions per second on a distributed test network. Graphene can easily scale to 10,000 or more transactions per second with relatively straightforward improvements to server capacity and
communication protocols.

Steem is capable of handling a larger userbase than Reddit. In 2015 Reddit’s 8.7 million users generated an average of 23 comments per second , with an average of 83 comments 13 per year per user. There were 73 million top-level posts, for an average of 2 new posts per second. There were about 7 billion up votes creating an average voting rate of 220 votes per second. All told, if Reddit were operating on a blockchain it would require an average of 250 transactions per second.