A few options.

* Keep in touch with past colleagues. Use linkedin. Congratulate them when they post something. This will take a few years.

* Go to 1 meetup/month. Talk to people there, but don't make plans outside of the meetup. This will take a few months.

* Write something regularly. Create a newsletter from it. This will take a few months to years.

* Post to HN anonymously. Keep making insightful comments. This will take a few weeks to months, depending on your level of insight.

All of these will grow your professional network without excess social activity.