Sending the first message is the bit most people stall on. You're writing to somebody you've never met, you've no notion how they'll take it, and there's no tone of voice to help you out. The good news is that it doesn't have to be clever. It has to be short, warm, and clearly meant for that person rather than for anybody at all. Five things make the difference.
"Hi, how are you?" gets deleted, because it could have gone to anyone. Read the profile properly and pick one thing to answer. A photo taken somewhere you know, a hobby they mention, a book on their list. Something along the lines of "I see you're from Clare. I did a bit of the Burren Way last summer and I'm still recovering." That could only have been written to that one person, and that's what gets a reply.
Three or four sentences is plenty. A long first message is a lot to answer, and it spends everything you might have talked about later on. Say hello, mention the thing you noticed, ask the one question, sign off with your name. There's no prize for filling the box.
A message made up entirely of questions reads like an interview. Give them a line about you so there's something to catch hold of. One sentence will do: what you were at over the weekend, why their photo caught your eye, what you're halfway through reading. You know yourself well by this stage of life, which makes this a good deal easier than it was at twenty five.
A bit of humour works well, and a light touch is best in writing, where there's no tone of voice to carry it. Gentle, and slightly at your own expense, is the safest ground. Slagging is grand between friends who know each other, but it lands very differently coming from a stranger in writing. Steer clear of jokes about their looks, and anything off colour in a first message. If you're not sure how a line will read, leave it out.
End with one open question they can reply to without much effort. "What part of the country are you from originally?" works far better than "Tell me about yourself", which is a big ask of somebody who doesn't know you. As openers go, the county question is hard to beat here, because everybody has an answer and most people have a story attached. One question, mind, not five.
A few last things. Read it back before you send it, because typos are easily made on a phone. Use their name if they've given one. And don't send the same message to twenty people, because it shows. If you don't hear back, that's normal and it's rarely anything to do with you. On to the next one, and keep it just as personal.