Meeting someone tonight is a different problem from meeting someone eventually. You need people who are available now, who reply promptly and who are close enough to reach this evening. This page is for last-minute plans, and last-minute plans work best when you keep them simple, public and flexible.
Send a short, specific message early in the day, ideally mentioning a time window and a general area. People are far more likely to say yes to 'around eight, somewhere near downtown' than to a vague offer. If they cannot make tonight, ask about another evening and stay friendly.
Keep your expectations loose. Last-minute plans fall through for ordinary reasons, and it is not a judgement on you.
A busy bar, a coffee shop that stays open late or a restaurant counter all work. Public, well lit and easy to leave. Let a friend know where you are and when you expect to be home. Arrange your own way home rather than relying on the other person for a lift.
If the conversation is flat or you feel uneasy, you can end the evening politely. Saying you have enjoyed meeting but are heading off is enough. You do not need to give a reason.
Look for profiles that were updated recently and say they are available at short notice, then send a clear message with a time.
It can be, if you use the same precautions as for any first meeting: a public place, a friend who knows your plans and your own transport.
Move on to the next profile. A non-reply usually means bad timing.