How many design professionals, in a moment of weakness, give in to subscribing to an online portfolio web site?
I'll confess – I have myself, and the grand total of referrals and inquiry, let alone business developed from them, is... $0.
The sites (portfolios.com and DirectoryM) aren't necessarily the most appropriate for a design firm to advertise on. Then again, neither are the yellow pages.
If you're a generalist firm, then perhaps they are a good option for your firm ( and then again, maybe not). But if you consider how many new clients you develop as a result of your web site, versus the relationships you nurture, then the answer should be fairly obvious.
A firm that specializes won't need a listing like that in a directory. Clients will seek you out as your expertise and reputation grows.
So... resist the siren song, so you don't get lost in the crowd of a directory. Invite your potential clients to come to your web site on your terms, not somebody else's.