AFAIK, Reddit, Slack, Dice, Bloomberg, IBM, Apple all use Solr. Jira and Confluence use Lucene. Others use Elasticsearch and Fusion (commercial product on top of Solr). See, for example: https://www.activate-conf.com/more-events for presentations from several past years on who and how uses Lucene/Solr.

Also, the new trend in jobs is "Relevancy Engineering", which is less about just setting up search engines and more on actually tuning them. That's where also Machine Learning and other AI techniques come in (Learning to Rank, Named Entity Recognition, sentiment analysis, etc.). Which was recognized by rebranding of the conference from Lucene/Solr revolution to Activate last year.

See also Haystack conference which focuses very specifically on relevance regardless of the specific search engine: https://haystackconf.com/