What is the best way to reach semi-retired or retired professionals?

If I were you, I'd try a mixture of Facebook (based on demographic features) and AdWords (based on search intent).

Additionally, you might accomplish a fair bit simply by networking with people in the right LinkedIn groups. If you can identify blogs with the right audience, then publishing an interview or advertorial might help raise awareness.

Curious what your brand is ...

At the PE firm I worked for, we often were looking for advisors to help on due diligence -- typically retired executives who could spend a few hours a week helping us on due diligence.

Many of retirees we worked with had LinkedIn profiles, and we were able to communicate with them over LinkedIn messages and then exchange contact information.

Feel free to connect with me on the phone if you would like to discuss further or have any questions!

Here are some effective ways to reach semi-retired or retired professionals:

Leverage alumni networks and professional associations. Many retirees stay connected to the professional organizations or alumni networks of their industry or alma mater. Reach out to these groups through newsletters, email lists, events, etc.

Connect at local community centers and activity clubs. Retirees often spend time at senior centers, golf and country clubs, recreational centers, etc. See if you can put up notices about opportunities at these locations.

Advertise on part-time job boards and portals. Sites like RetiredBrains, SeniorJobBank, Seniors4Hire, etc. specialize in flexible opportunities for mature workers.

Attend retirement seminars and financial planning events. There are many local and community seminars about transitioning to retirement that attract retirees.

Partner with retirement coaches and planners. Retirees often work with retirement coaches and financial planners; consider outreach to these professionals to access their networks.

Run ads in print and digital publications focused on retirees. There are many magazines, newsletters and websites catering to retiree interests that you can advertise in.