Is a Social Media Presence Relevant in this Industry?
Yes! Particularly as it relates to branding yourself. However don't spam people, repeat yourself incessantly and pat your own back.
In my opinion some delivery channels are more forgiving than others. If you tweet several times a day it's not as disruptive as --for example-- posting to LinkedIn more than once a day. It makes sense to consider the channels you want to use as free advertising and do so with the mindset of what you as a consumer or audience would appreciate hearing from a service provider.
No matter where you are sharing, it's most important what you are sharing - is it relevant? For example - are you tweeting about your cat? If so, unless his voice is on a Fancy Feast commercial maybe save that for your personal Facebook page. Do you give and take? For instance if your colleague in Lisbon just landed a McDonald's commercial over there -- it's nice if you share the joy - let others know, congratulate him... All these are ways of giving back, just as you would in any community.
If you are savvy enough to know when you are mentioned (there's an app for that), respond to those mentioning you. Offer free advice when you can.
In terms of who you follow-- it's nice to be in a community of voice overs but you also need to think of who gets you jobs -- try following those who hire, voice studios, agencies, production companies and the like...
I am not great at practicing what I preach but you might be better at practicing what I preach -- post consistently, themed, with a presence that is recognizable, be targeted and deliberate in what you choose to share and how often you do.