Joseph-
In both cases you give, I think it would be extremely unlikely that an individual with ulterior motives would know frum lingo and be so driven to put on such an act.
In the case of the runners, they can meet in, let’s say, Mendelson’s, with male relatives present. Alternately, find common friends or acquaintances. I’m sure there are other ways as well to filter out creepy poseurs.
In the case of someone who claims to be a shadchan, let him/her provide proof that they are indeed what they claim to be, via references, a known Rov, etc. before even considering contacting them.
Please realize that the “Ride Sharing” thread is already allowing contact for a good reason, even though a stranger in your car is potentially more risky – obviously in all these cases common sense and caution are important.