Some ideas:
- mailing lists (wikitech-l, mediawiki-l/mediawiki-announce, mediawiki-api/mediawiki-api-announce, wikitech-ambassadors, possibly anything else with a decent number of subscribers)
- banner campaign (mediawiki.org, wikitech, possibly other wikis?)
- messages to technical village pumps
- Tech News
- Phabricator?
- gerrit?
- MediaWiki Facebook + Linkedin group(s?)
- Paid ads? (Stackoverflow can run tag-specific ads; Linkedin, Facebook can run skill-specific ads)
- IRC messages?
- direct emails based on Phab/gerrit/whatever account information?