What if a long-established agent (several bestselling authors) with a well-respected agency has no social media presence? Personally, I'm okay with it, but several of my writing buds see it as a red flag. Could it be?
An agent doesn't need to have any presence on social media to be effective.
A robust presence on social media is not indicative of a better agent than someone with fewer tweets, or posts.
For some agents social media is fun. It's a way to stay in touch with clients in a low-key way; it's a way to keep up with authors who aren't clients. It's a way to help queriers get a sense of what the agent is looking for. It's someone to talk to at 3am. It's a place to find fellow subway rider sufferers.
But social media has a very very dark side. A casual tweet, carelessly posted can resonate in ways the agent didn't imagine...and hurt his/her public image.
People can say things to and about agents that are casually cruel (intentionally or not) and it's hard to fault anyone for wanting to avoid that.
There's a big difference between understanding how social media works and being present on a social media platform.
You want an agent who understands social media, both its strengths and pitfalls, not necessarily an agent who has learned those strengths and pitfalls the hard way.