I've been up to ~8000 mass and #1 for a good stretch. Longest game was 90 minutes.
A really good tip is to gain some allies and help each other. Help them out when they are struggling and they often help you out back. Protect a smaller player, keep trading food. If someone is after then bulk them up, use them to corner other prey. Pick a memorable name so if you do help someone out they may remember you.
The best advice is not to split unless you are very confident it's safe (no one big close to you) and it's well worth the risk (making a substantial increase in mass). Don't split for small gains, just be patient.
Also, if there is someone big after you then you can eject mass to speed up a bit and then eat it up again as you go. Or as a last resort split into lots of pieces ... you shrink down but get to live again.
You can also fire viruses and break apart large cells ... eject mass (W) in the virus direction and after a few hits a Virus will shoot out, fragmenting anyone on the other side. Great if you are moderate size and there is a big cell after you. Of course if you're big, beware of the virus cells.
Beware of going after people in two pieces where you are larger than one piece but not the whole - if they manage to re-merge before you consume one of them then they can turn and devour you. If someone big is after you and you think you're done for, start feeding someone else who may be become big enough to eat your attacker - they'll often save you and let you live.
Be courteous, if someone helps you out then send some food their way.
Mobile is unplayable compared to the mouse. Lookout for players who look like they are on mobile (you can tell by the movements or complete stops). They are just asking to be eaten.
Spectator mode is good for learning tactics.