PROS
* Potential marketing for your startup, especially if you gain followers that are also in your target audience.
* Have a way for someone (e.g. investor, potential employee, potential business partner, etc) to look you up and see what kind of person you might be.
CONS
* Distraction away from your customers and team, which should be among your top priorities. It can take time to cultivate a social media presence.
* Misinterpretation risk. For better or worse, if you happen to say something that can be misinterpreted, and someone has an agenda against you, social media can be risky. Assume that everything you say can be posted on a billboard in every major city.
MY $0.02
Each of these carry different weights. I would argue that the disadvantages far outweigh the advantages.
If you want marketing benefits, you'll get more bang for your buck by having someone with lots of marketing experience. Being discoverable by investors via social media is really a minor thing; you'd have to substantially invest in your "social media reputation" in order for this to give you anything, and even then, it's more useful as a tool to maybe get meeting with an investor, than it is to secure funding from that investor.
Being a distraction from your core priorities is really the most important thing I'd offer. Social media can easily be a black hole for attention and time. That's attention and time you can't afford to lose right now.