I'm gonna be brutally honest mate, as a fellow POF user, don't get your hopes up. The average rate of reply is one reply for every 30 girls messaged (Taken as a general rule from the POF forums).
It's a very shallow place, where people are always looking to get above their league, hoping that personality shinning through online will over ride physical appearance. I'm guilty of this myself, as I'm sure you will be, it's easier to simply ignore those you are unnatracted to if they message you than give them a glimmer of hope then have to turn them down later.
From both my own, my brothers and one of my friends experiences, no one is on there just looking for friends. It's a way of gently letting you down later after probing the relationship waters. Out of my, admittedly limited, circle of friends/people I know, most of us use or have used it at some point. My brother has managed a couple of ongoing text conversations lasting a few weeks, as has another of my friends. Another guy has managed a couple of dates that went nowhere and one girl has met up with a few guys, but they tend to lose interest after getting what they want if you catch my drift.
I'm down as big and tall/bbw, the biggest option they have (because I am a fat *******) so I don't mean offence when I say this, but the fact you've got yourself down as a few extra pounds will hurt your chances. When you view a profile it's pretty much the first thing people see in the top left corner, and many click the back button as soon as they see anything more than average. At least you have been honest, my brother and I find it quite amusing the amount of 18stone + women who put themselves down as average.
I've been blunt with you because POF can be a depressing thing. When forming relationships elude you in the real world, as they do me, it's hoped the Internet can be some last hope. I've been on it about 2 years now and I've received maybe 15 messages in that time, none of them from women I have personally found attractive. I sent a few messages initially, but after realising I fail online aswell it knocked what little confidence I had left.
If you can take the inevitable rejection I'd say message everyone you like, even those you are kinda "meh" about, something has to stick. To use a poor analogy, I treat it as if I'm a sniper, I carefully consider a person, craft a well thought out message, then fire. Unless you're really good you'll miss, then you have to line up another shot. I do this because I like a specific type of girl and I'd say 90% of the women on there are very mainstream and not my type. Those who seem to have any degree of success take on a more LMG approach, send as many messages as you can, you'll miss alot but you'll hit something at some point.
If you have any luck at all then please, please tell us how you managed it.