Back when I had an active OkCupid account, I'd run across a profile or five that would say they were in a relationship but the member was online within the last week. Other profiles would say they were only looking for friends or long-distance pen pals.
When my ex and I first started dating, he said he kept his profile mainly to "troll" or keep in touch with interesting conversationalists. I didn't mind so much because he was listed as "in a relationship" and I trusted him.
My roommate met her current boyfriend on the dating site and was more than peeved when she saw he was still logging in and listed as "single" months after they started dating.
I always thought that those who created profiles on dating sites just to meet friends were more those who didn't want to rule out relationships completely but also didn't want anything resembling a commitment right away (but they didn't want to check "casual sex" under "what I'm looking for" either). Murky grounds they be.
Would you ever join a dating website merely to find new people without any motive to date them? What do you think of those who do join dating sites seeking platonic friendships? Would you be upset if your new SO continued to peruse a dating site?