I haven't read all of the replies although most seem to be misinformed about one thing. While there is certainly nothing stopping the steemit team from writing their own algorithm, the current algorithm used is not in the steemit codebase.

Below is the call in condenser that gets the trending posts already ordered from the steem node.

if (order === 'trending') {
  call_name = 'getDiscussionsByTrendingAsync';
    args = [
      {
       tag: category,
       limit: constants.FETCH_DATA_BATCH_SIZE,
       start_author: author,
       start_permlink: permlink,
      },
    ];
 }