Is it worth it? Of course this depends on how important a presence on the web is to the startup. And it also depends on the market sector.
My own experience is that an agency can get thousands of followers fairly quickly. That can really help, when startup approaches a potential affiliate, potential investor, potential client. It can give the business credence when reaching out for more business.
There's a creative element to marketing. Marketeers should be able to create ideas or present your business with regard to attracting interest. It's not easy for all start-ups to be able to pitch effectively because the start-up generally requires a huge amount of time just to get the business started, and that can blinker the creativity aspect (unless someone has a knack for marketing AND the time to implement).
My advice would try to get a personal recommendation, or at least look on the web to get a feel for what type of agency appears best suited for your business.
